I think I'm using "Allied Forces" and "Europeans" too loosely. I was lumping Ukrainian forces into that label. They're already under attack so any conventional retaliation on invading forces is already a given. I'm basically saying that NATO/EU would put pressure on Ukraine NOT to attack the staging area because any fallout could waft over into Western Europe. This is in addition to the already present downside of Ukraine re-contaminating their own backyard.