Both sides could have respected the Minsk agreement.

NATO could have resisted expansion to the East.

A federalized option agreeable to the Ukrainian secessionist provinces promising some autonomy might have been successful.

The West could have declared an eastern European buffer zone between NATO and Russia. ("Little Entente" v2.0)

Some sort of internationally-monitored plebiscite in the Ukrainian secessionist states and Crimea.

There's thirty plus years of historical context that's boiled over into a shooting war, but that context is important for understanding where we are now without relying on simplistic abstractions like nationalism or "democracy".