How can international security agreements be enforced when one party fails to honor them?

I was asked that on Quora, and while it’s a short piece, the answer is, I think, worth posting here for all to read.

unless you want to go to war over it, they can’t. The Ukraine is the perfect example. Russia was a guarantor of Ukraine’s security and safety. So was the US. All of this in return for Ukraine turning over to Russia or decommissioning the n…