Historically many if not most conflicts started with the breach of an agreement. Without getting bogged down in irrelevant detail, there are issue of self-determination of Crimea, which repeatedly in 3 referenda (2 if you wish to exclude the last one) pronounced in favour of either autonomy or being part of the CIS (effectively Russian Federation). Likewise, and setting aside the 2014 events for the moment, there also were agreements that, in principle, may have served as a valid status quo, such as Minsk II, and were not complied to by the parties.

So, sure, some form of trust-building will be necessary. But what's the alternative? Endless war?