Ukraine is far from a forever war, unless you're proposing that Russia will keep invading them forever. They're literally defending against a foreign enemy, you know, that thing that all soldiers swear to do. It is beneficial to Ukraine and to the United States for Ukraine to stand. If Russia expands into Ukraine then we're back in a pre-WWII situation where a foreign enemy power is expanding across Europe against the will of every other country on the planet, except maybe China and NK.