After playing the tutorial your first duty in the campaign is to assemble a starting team.
There are no char classes in this game, but some skills, attributes and traits are more for combat, others more for support.
The mercs differs in price, level, attributes, skills, traits and friendships.
You have to find a balance between the needed damage and the needed support quality.
Take your time to plan your starting team, because you keep this mercs for the rest of the campaign.
After making enough money you can hire additional mercs in the campaign.

Price
Unfortunately you are starting with a limited budget (40K) and can't afford the best mercs.
Some mercs like Spider and Igor are not ready to join you, even you could afford them.
You pay for your mercs just the one time you hire them, there is no additional paying.

Traits
Most traits are self explaining.
The description for Amazon says: Morale rises when working in a squad with only women.
This is wrong and the effect is negative, the morale lowers if a man is in the same squad.
Some mercs like Meltdown and Fidel have the Loner trait. This can be simple achieved by splitting your squad.
This unnecessary splitting in additional squads because of traits is very user unfriendly, and you have to win the first map before you can do it.
After the first quarter of the game it makes sense for the Handgun experts to use another kind of weapon, because sub machine guns and shot guns are clearly better damage dealers at the same range.

Morale
A quote from the outdated strategy guide of the developers:
Morale is a lot more of an issue than keeping a team of happy campers. Each merc can get a bonus or penalty to all their equipment checks based on their morale. A merc's morale has a default value of normal that has no positive or negative effect on them.
However, it can rise or fall to good, very good, low, and very low. Each tier represents about a 10% bonus or penalty, ...
I can't see the values normal or very low in the actual version, I just found the following three different values:

• very good = gives you the bonus
  
• good = no change.
  
• low = gives you the penalty

A good example is soundless, normally you need 100 agility and 100 stealth to reach this goal. With the bonus (very good) you need just 91 agility and 91 stealth for being soundless (or a summated value of 200 of agility, stealth and moral bonus points).

Friendship / Personal Animosity
Some mercs are changing their mind because of the team members you have already chosen.
Igor joins you when Grunty is in your team and Fox refuses to join if Steroid is already there.
A friendship also improves the moral of your team member, if they are in the same squad. Personal animosity lowers the moral.
Both emotions can be one sided. You can hire Steroid after hiring Fox, but not the other way.
You can hire Fox and Buns in a starting team and there is no problem. But when I tried later in the game to hire Buns she refused because of Fox in the team, that seems a little bugged.

Melee Combat
Normally you try to avoid melee combat and shoot the enemies. Only exception is the sneak attack on a single enemy with a melee weapon. This makes the traits Melee Weapons, Melee Mastery and Tough nearly useless.
For all fans of Bull and of the original JA2: There is no big fight for him in San Mona.

Marksmanship
Determines in combination with dexterity, stance and the range of the weapon the ability to hit the enemy. Even a bad shooter has a real chance to hit an enemy if he shoots lying on the ground and the enemy is less far away than half of the weapons range.
Some traits gives you a bonus for a certain kind of weapon.

Supporting Skills
There are three kinds of support and for each you need a specific basic skill minimum to use the important tools:
M) Medical for healing team members
C) Mechanical for repairing weapons
E) Explosive for removing mines
You should have a merc with the medical level M3 and a merc with the mechanical level C5 in your starting team. Soon after the first maps you also need someone with the explosiv level E2 for defusing land mines.

Medical

Craftsman (Mechanical)

Explosiv

Possible teams covering all 3 supporting demands
Fox is a precondition for hiring Nails.
Because of their animosities you must hire Fox before Steroid and Buns before Red.

My personal starting Squad is Wolf, Fox and Grunty.
Wolf is a very good mechanic and an all-rounder, because of his traits is his specialty to storm into rooms and fire with the shot gun from the hip.
Fox gives Wolf and herself the moral bonus, is a very good nurse and she is cheap. On the first maps she can make sneak attacks from behind. If you develop her agility and stealth to 91 she is soundless.
Grunty is for a starter a good shooter, has some medical and mechanic skills, is the invitation card for Igor and Ivan and starts with a MP5K for Fox.
My first additional hiring is the explosive expert Barry Unger.

Comments to some mercs in JA Back in Action
(In JA Crossfire some mercs have changed in price, level, stats and gear)

Bull 9,830$, Grizzly 14,670$: Both have low marksmanship combined with low dexterity, bad support quality and traits for useless melee combat.
Fox 9,860$: She is a very good medic (level M4) and has with Sub Machine Gun (expert) a good trait, but a starting marksmanship below 50.
Spider 9,959$: She is a great medic (level M5), but she is not ready to join you at start and has the very bad trait Amazon.
Grunty 11,250$: Has some low supporting qualities (M1, E1, C4), 70 marksmanship and a very good starting weapon (MP5K). The trait Machine Guns is for the second half of the game.
He is also an invitation card for Igor and Ivan.
Buns 11,388$: She is a good medic (M3) and with marksmanship 73 and 78 dexterity a very good shooter for her price. The value of the trait Handgun (expert) is great at start and then decreases.
MD 11,660$: Is a great medic (level M5) and has a good starting weapon. Because of his trait Wimp he should stay behind other mercs.
Igor 12,316$: The trait Pathfinder makes the whole squad faster on the world map. A good merc for later hiring, because you have to hire Grunty first.
Steroid 12,350$: He can repair all weapons (level C5), but except for strength his attributes are low. With his Macho trait it's easy for him to get the moral bonus.
Barry 13,065$: His very good explosive value of 78 (level E3) gets boosted by the trait Explosives (expert). In the other supporting fields he has some knowledge (M1, C4). Because of his trait Wimp he should stay behind other mercs.
Red 13,430$: His explosive qualities are similar to Barrys. He has a better starting weapon and has slightly better marksmanship. Most of Barrys attributes and other skills are better. Red needs glasses to compensate the trait Short Sighted.
Malice 14,295$: He has a marksmanship of 73 and high 92 dexterity, but no support qualities and gets no boost from his traits. So he is not bad and not really good. He likes Fox.
Hitman 15,640$: His low 40 dexterity is bad, because it's needed for aiming. He has no support qualities, but the starting weapon is nice.
Wolf 17,684$: He can repair all weapons (C5) and is a good medic (M3). All his attributes are good and his traits Gunslinger and Shotgun (expert) are a perfect combination for CCQ.
Thor 18,775$: He has good values for sneaking, but his price is high for being just a very good medic (M4) and having a marksmanship of 64. He has no useful starting trait.
Meltdown 20,030$: As an Explosive (expert) (E2) she is not as good as Barry or Red. But she is a better shooter and her trait Machine Guns (expert) is for the second half of the game.
Nails 20,606$ He can repair all weapons (C5) and remove land mines (E3). He has a great starting weapon and the good starting trait Shotgun.
Raven 33,400$: She got all a good shooter needs: 90 marksmanship, 90 dexterity and 94 perception. Her trait Sniper Rifles fits right in. She has no supporting skills. She is the most expensive merc you can hire at start but you do not have the money to hire an additional merc.
Among the high quality shooters she is the cheapest so keep her in mind for later hiring.
Ivan 53,286$: Our hero from the very first JA is still a boost for every squad and can remove land mines (E2). He is too expensive for a starting team.
Reaper 58,675$: The merc with the best traits Assault Rifle (expert) and Sniper Rifles (expert). With Shadow in his squad he gets the moral bonus, then he is soundless and a real killing machine.
Magic 64,690$: He has great attributes and is the best mechanic. The great trait Assault Rifle (expert) compensates his marksmanship value of 75. To make him soundless you need just 6 training points or the moral bonus.
Fox, Grunty, Buns, MD, Barry, Red, Hitman and Thor have the trait Team Player. This is a good trait later when you have a full squad.

Double Sided Friendships
Some mercs (like Blood) are not first choice but in combination with a friend they give themselves and the friend (Magic) the moral bonus. Magic with moral bonus and a G11 is awesome.
Wolf <-> Fox
Grunty <-> Igor <-> Ivan
Raven <-> Raider
Magic <-> Blood

Stance
The four possible stances for a merc are:
1) Run
2) Ready Weapon
3) Crouch
4) Prone
From 1 to 4 the shots are getting more accurate and slower.
Shotguns and handguns can even in run mode be useful.
Sniper rifles and machine guns are nearly useless in run mode.
In the plainning mode (space bar) you can change stance and aim at an enemy. Depending of the planed stance of your merc and his abilities the time and the possibility for the shot is displayed.
The noise of moving mercs is also dependent to their stance.

Tactic tips for the first battles
Best tactic for starters is to wait on your knees for the patrol behind a corner. In long range battles you can be out of ammo after the first shootout.
If you have stealthy mercs in your team use the sneak attack for standing guards.

CQC (close quarter combat)
A merc with a shotgun, sub machine gun or handgun should storm into the room and into a position where he can fire from the hip but the enemy can't counter with melee.
The stance should be Run or Ready Weapon. Attach a Holographic Sight for time bonus instead of a scope.

If there are more than one enemy in the room you have to adept your tactic.

• Finding a position where only the target can aim at your merc.
  
• Throwing in a stun grenade first so that the enemies are stunned.
  
• Make the enemies leave the room with grenades, noise or just ran in sight and back.
  
• Use all entrances of the room and make a combined attack (don't forget the windows).

Wolf is the specialist for CQC (close quarter combat) because his traits are made for this: shotgun (expert) and gunslinger.
Nearly every enemy is down after a burst with the CAWS.

Original JA2
Don't expect this game to be exactly like the orignal game with modern graphic.
Compared with JA1 the original JA2 and Unfinished Business made many improvements, some of them are ignored by this remake. Best example is repairing.

Tactical Mode
When you choose to start the game with enabled Tactical Mode stealth and perception are getting much more important.
In this mode the game play is more like the original JA games.

Team / Squad
Later your team can grow with additional hiring or getting recruits after solving quests.
If you have more than 6 soldiers, you must have another squad, because for a squad there is a limit of 6 soldiers. You can split your team in as many squads as you like, but only on the world map.

Level
Your soldiers can raise to a maximum of level 10. For each level up you get 7 training points for improving attributes and skills.

Stealth / Sneaking
The stealth skill determines in combination with the agility attribute the noise a moving soldier makes.
When the game was released there was no such thing like 'almost stealthy' near enemies, it was just digital. Now some mercs can sneak up to enemies on the first maps and make an one-hit kill with a melee weapon like a knife or machete.
For being soundless the soldier needs 100 agility and 100 stealth. With morale bonus it's reduced to 91 agility and 91 stealth (or a summated value of 200 of agility, stealth and moral bonus points).
If you are not going for soundless it doesn't really matter what stealth skill the soldier has and then don't waste any point from leveling to stealth.
Spoiler for finding items on the first map: At the watch towers outside the fence you can find a silencer and a machete for sneakers.
Fox, Meltdown and Thor have for possible starters good sneak abilities.
Later in the game:
You need to be soundless for successfully sneaking near enemies, even there is a big wall between your soldier and the enemy.
Some enemies are triggered by proximity, but they still don't know about the presence of a soundless sneaker. The biggest advantage of sneaking is to get into good firing positions, before the enemy can react.

If you have not yet freed the airport Drassen, then you should do so before reading any further.

Weapons / Ammo / Armor
(click on the picture to enlarge)
Throughout the game you will find better models of each kind of weapon. It's important to improve the mercs with better weapons, but never forget to have enough ammunition.
Some weapons seems to have similar data, but the different ammunitions can make a huge leverage.
The armor piercing value is getting more and more important, because many enemies are armored. The three most important characteristics are range, kind of ammunition (damage) and rate of fire.
Spoiler for finding much better weapons: On the local map Water Pump southeast from Drassen there is in the north a building without enemies where you can can find two long ranged weapons. At this stage of the game they are awesome and the only problem is to have always enough ammo for this weapons.
There is also a bag of diamonds, which can be sold or used later in a quest.

Militia
Don't waste your best weapons to the militia. Give the best weapons to your team, the cheapest to the militia and the rest sell to the traders.
Repairing Weapons before selling gives you XP and a better price.
From time to time you should save and then speed the world map, so that you can see where the next attack of the enemy will be and you have the chance to welcome them.

World Map
On the the world map the clock is running, but by using design flaws you can enormously reduce travel time and travel fatigue.
The really bad thing is, that the designer expect you to use this flaw. If you do not, the game is much harder.
When you can enter a local map on your travel you should do so, because on local maps the clock is always stopped. The travel on the local map also changes your position on the world map.
Always go to the rim of a local map before changing to the world map.

A thread about how the game is getting harder without flaw using:
http://steamproxy.net/app/57740/discussions/0/846961716422566719/

An extended guide about the time management issue has been written already:
http://steamproxy.net/sharedfiles/filedetails/?id=159307111

Normaly your first game is JA Back in Action, but the dlc JA Crossfire can be played without having JA Back in Action.
http://steamproxy.net/steamstore/app/205810/
The mercs have changed a little bit in Crossfire, but most other things are the same and described for booth games in this Guide for JA Back in Action.

Comments to some mercs in Crossfire you find in my guide for JA Crossfire
http://steamproxy.net/sharedfiles/filedetails/?id=206731835#204171

There are 6 other DLCs for JA.
Two of them contain a new map. You can't play these maps with your usal team, because the mercs for these missions are already choosen.
http://steamproxy.net/steamstore/app/203345/
http://steamproxy.net/steamstore/app/203344/
I bought them on a weekend with 75% discount and enjoyed both.

The other four DLC have been special selling bonuses for different vendors.
Each of them contains 3 camouflaged items: a rifle, a helm and a shirt.
You get the items in the game after returning to the airport.
I think I've got the Urban Kit because of buying the game at release. The actual single price of 1.49€ for each is totally overprized.

• Urban Specialist Kit
  
• Night Specialist Kit
  
• Jungle Specialist Kit
  
• Desert Specialist Kit

Other developers made an attempt to build a remake of the old JA. But it is at the moment (november 2014) still unfinished.
http://steamproxy.net/steamstore/app/256010/
http://steamproxy.net/sharedfiles/filedetails/?id=329912211

http://steamproxy.net/steamstore/app/283270

http://steamproxy.net/steamstore/app/215930/

https://steamproxy.net/steamstore/app/1084160/Jagged_Alliance_3/
https://steamproxy.net/sharedfiles/filedetails/?id=3007621379

Please, no curses, insults or links in the comment area.

I hope this guide helps you to keep your mercs alive.