“What treaty?”
The Russian-Ukrainian Friendship Treaty of 1997.
“The treaty consequently expired on 31 March 2019.”
Ukraine chose not to renew because Russia seized Crimea and had little green men throughout the eastern provinces. Russia broke the treaty, but claims Ukraine broke it when they stated a desire and aspiration to join the EU and NATO.
A major element of the treaty is that neither party would do anything to lessen the security of the other. It’s not a stretch to claim having one’s decades long, existential enemy invited to exercise next door is a violation.