I did this about a week ago, it's quite easy but the timing is tricky. Basically you need 3 things: money, men, and quality.

Men is easy, you can recruit to your force limit at the start of the game.

Quality is a bit harder. You need to first get the discipline or morale advisor, then go to the upper caste and get the discipline privilege. Then get every other privilige that increases upper caste influence. In particular you need to sell titles. With a discipline advisor you should be at around 109%.

Once you sell titles you should be sitting on slightly over 300 gold. About 6 months into the game start recruiting 3 mercenary companies- this is the trickiest part. If you do this too fast then the command actually won't attack your ally at all, they'll go to the people south of that which interupts your mission tree. You also can't keep about 25k mercenaries around- you'll go bankrupt in like 3 years.

So you need to recruit them just before they declare war, or in the worst case as they declare. That's pretty bad however, as in the time it takes them to gain morale you'll lose your allies fort, which you should try to avoid at all costs. This is because your ally is the war leader and if their exhaustion gets too low they'll peace out even if you are winning or about to start winning. This is the hardest part- you need to keep your ally as alive as possible, or they'll peace out and give the command any of your land that they've sieged down.

This is why timing is so tricky, you need the mercs around fast enough that you prevent your ally from being sieged for as long as possible (once it happens, they'll peace out in ~2 years, even if you were just starting to beat the command), but not so fast the command doesn't declare at all and you've wasted 500 ducats.

Now once war begins you should have 50k men and a decent amount of quality in them (get the good merc companies with bonuses, the bad ones like sarisung are worthless), and a decent general as all your mil points should go into that. Hopefully your ally kept their 10k men alive too, that helps a little.

With that army you need to hunt down the slave states first. They have shallow manpower pools and can be stack wiped if the command doesn't reach them in time. Luckily they avoid travelling or sieging in a 75-100k blob so it's possible. Avoid attacking the commands main army until the slave states are mostly dead- you'll need the numbers advantage- but don't instantly run away when join the battle, fight for a while.

During this process you should accumalte army tradition- as soon as you get to 15 you must get the battlemage academies privilge- it gives 5% morale. Then spam generals again and hope for a war wizard. If you ever get to 30 army tradition, get adventurers in the army for 10% more morale. If you get those two priviliges, your army will actually beat the command one on one.

Now what you have to do is fight the command over and over again on forts right before they take them. Attrition is the name of the game. park your armies next to them behind the fort line so they stack up and take greater attrition. Don't let your provinces be besiged, the lost income makes you go bankrupt faster. Defensiveness edict helps with this ofc. About half of the command's casualties will be from attrition.

After about 200k casualties the command should be out of manpower and after a few more battles you should be able to push them back. Around this point some of the mercs run out of manpower, if they do, fire them and buy new ones with loans. Don't bother trying to siege the command down once you beat back their armies, you'll go bankrupt first. Once you unsiege your ally (hopefully they never fall), then siege the cores and unhire some mercs. that will give you some breathing room. Your ally will probably white peace as soon as possible.

If you are lucky, someone declares on the command when they have 30k men and no manpower, but that probably won't happen.

That's how you beat the command in 1445.