[quote=@Rare]
So, the war could still have got on; but, Poland didn't annex Ukraine.

I mean, whatever is the best. You're the GM after all.
[/quote]

No, political union would still have occurred. That's cannon. What's up in the air is the details of the war. How long it lasted, how deep Hungary got before Poland came in, and more importantly: why it happened.

I think a few assumptions could be made about the status of Ukraine post-Empire. With the collapse of the Imperial regime in Moscow, Ukraine lost the resources needed to really defend itself. The fall of the Empire made them independent, but it didn't give them a foundation and it was sudden. Five years on and things aren't all that great for them with no foreign aid helping. So invasion happens and they get their ass whipped.

At some point the situation gets so threatening Poland steps in to keep a regional balance of power. Being fresh they managed to reverse the Hungarian invasion, and supply the beaten Ukrainian militia-army enough they can do more. Together they throw Hungary out concessions are made, likely enforced by Poland.

Presumably Poland having saved them from the dirty imperialist Hungarians would probably make union popular, and it happens in the events following the invasion.

In all I imagine it could play out across a year or year and a half maybe with a few significant urban engagements before the fates re-align.

Though probably not canonical anymore Ukraine briefly occupied the Moscow region of Russia but gave it up to the Republic in Yekaterinburg at some point. This could be framed in the context of before, during, or immediately after the war.