This guide is old and not really needed- You can still follow this guide if you want but honestly you should be able to win with just the starting units and using all of your Close Air support sitting around in Italy. Still I endeavored to write all this crap up a long time ago, if you want to read it go ahead!

To defeat Ethiopia in 140 days before they can go into exile it's actually rather easy with the right setup. It might take you 1 or 2 attempts to get it done consistently but using these steps will ensure victory!

First of all, upgrade the railway between the port and supply hub to level 2

Now for your armies. Assign both armies in africa to the field marshal with the "organizer" trait. For the eastern africa army, setup an offensive line like this and set them to aggresive.

Now for your northern army, assign all those troops an offensive line like this. No need to change aggression levels or cohesion yet. I highly reccomend using the general Ezio Rosi. He's an infantry expert and an organizer, so he's good to finish the push once you assign him the infantry expert trait, and later down the line you can promote him to a field marshall for logistics wizard.

Now we are going to grab some troops from the mainland. Grab your 4 mountaineers and 2 fully trained basic infantry divisons. Set the frontline on Aussa. Delete all of your other troops except for the 2 undeleteable divisions in Libya- you need equipment for you frontline troops ASAP.

Now for the final preparations. Set your occupation law to Local police force, and prioritise garrison and reinforcements for equipment.
Now, motorise your field marshall! Italy starts off with some trucks and are building more, you can afford it and you will need it. Set up your airforce, Bring all your tactical bombers to Ethiopia. Put everything in the north except the 64 air wing division, you can put that one in the south.
I reccomend using your mills to have at least 10 mills on guns, 3 on artillery, 3 on support equipment, and make sure you keep those 2 on trucks.
The focus order should start with Ethiopian war logistics. You can take any other focus' you like after that, but just know that after War logisitics finishes, you have 105 days to defeat Ethiopia.
Now, begin the offensives with your 2 armies! When the rest of your airforce from italy arrives makes sure you set them up!

With the help of the rest of the airforce arriving you will begin to push. When this event comes up take the top event for increased attack and air support. You have to win the war in 140 days from the start of the game, no time for supply!

When the event to annex Aussa fires, annex them, then combine your frontline troops on Aussa with your northern army. Draw a new offensive line!
Your eastern army is advancing well enough. Now with a little bit of micro, victory will be secured!

When command power is available, give your field marshall logistics wizard once he reaches level 2!
When command power is available and Ezio is now an infantry leader, assign him Infantry expert!

Micro time!
You may want to change the cohesion settings to balanced for your northern army now so they don't constantly relocate from left to right. be aware that this may leave some gaps in the eastern line, so you might not need to. You should, however micro some troops on the left hand side and grab some empty Ethiopian land before any additional troops show up. Also, make sure you re-assign your planes from the northeast air zone since you control all that land now!
Watch your frontlines to make sure there aren't any big gaps. The goal now is to secure the supply hub in the east, and then attack Addis ababa from multiple directions. Once you have Addis ababa, the rest of Ethiopia will quickly fall before the 140 days is up.
as soon as you get that supply hub, connect it to your northern supply hub via railway. Usually you end the war before its fully built but do it anyways.
With some micro, but letting your generals still battleplan, you should be able to take Addis ababa via micro from multiple directions, and then link up to your eastern army. At this point you can just delete your generals orders and use a field marshall battleplan on aggressive to clean up.

Victory! if you see this screen, you defeated Ethiopia before the emperor went into exile, ensuring you don't have to deal with the pain that is crazy amounts of resistance, as well as technically still being at war!
Italians don't like the U.K for the setup of the government in exile, but the emperor never actually made it to the U.K, so you did everything right!
I did the focus' Ethiopian war logistics --> Italian Highways ---> Power plants in Terni.
The war should end before your 3rd focus is done (or 4th focus if your 3rd focus you took was also only 35 days) If it isn't over by then, Ethiopia will auto capitulate since Boarding the train lowers their surrender limit, but lets them go into exile.

Nonetheless you did it! Ethiopia is all yours! Well except for one little hiccup. You are going to get the missionilini: Pacify Ethiopia.
I don't think you can actually do that, without Ethiopia still being technically alive in exile in the U.K like they are if you didn't finish the war in 140 days, you don't get access to any decisions to bring down resistance, but don't sweat it. Failing pacify Ethiopia only loses you a miniscule 25 PP.

Now go forth and conquer!

I made a small guide on what you might want to do with Ethiopia in the peace conference, read it if you want! Or don't, in which case I spent 2-3 hours on it for nothing

https://steamproxy.net/sharedfiles/filedetails/?id=3190691521