It obviously wouldn't be a declaration for war for the following reasons.
1. Ukraine is an internationally recognized sovreign state.
2. It has a democratically elected President and cabinet.
3. Its leaders can invite NATO (or any of its individual member states) to create a no fly zone, a humanitarian corridor, or similar and it would be legally recognized.
In all practicality, Putin would view this as an act of war, but excepting the use of nukes (which would guarantee his own destruction), he wouldn't have any power to do anything about it.