If the point of supporting Ukraine is to support the international order of respecting borders, then an absolutist interpretation would mean you stop at your border when repelling invaders.

On the other hand, that would certainly result in invaders loading up on personnel and materiel on their side of the border until they reached some critical mass for a re invasion.

A lot of people might not remember the first Gulf War where the international community defending Kuwait stopped at the Iraq border. I think it could be argued that was a mistake on multiple levels, even ignoring everything we know that came after.