[HWR_14]

I assume you mean downsides geopolitically, because the financial costs are sunk either way.

There are several losing states (complete annexation, split like Korea). However, there are only two main ways Ukraine can lose. The world can stop sending free weapons or even with free weapons Ukraine is defeated.

It's hard for me to think of any way that "the west stops sending weapons" is not more likely to result in Russian or Chinese aggression. Is China more emboldened by "US arms without US military personnel lost a war" or "the US can be counted on to lose interest in any conflict over 2 years"?

So it doesn't matter what the downsides are[0]. It seems like no matter what, the US is better off helping Ukraine.

[0] Unless Putin pushes the big red button. But that topic has been written about extensively.

[MichaelZuo]

It seems possible for the Russian decision makers to be so angered that they will want to take revenge, to spend their built up political/technological/social/cultural capital to intentionally weaken the US, EU, etc.

Nuclear weapons are practically an all-or-nothing proposition, but nowadays there a lot of other ways to deliver some intermediate amount of 'revenge'.

And it's a lot easier to destroy then create, as the war has shown, I estimate the asymmetry is easily 100 to 1. That is to say $1 of novichok, or whatever nasty means, can easily cause $100 worth of damage, and the combined economic size difference isn't even 20 to 1.

Though I'm just spitballing the numbers here.