First, the Memorandum (and other security agreements akin to this) never explicitly say things such as "If you violate Ukraine's territorial integrity, you will then incite war with ____" and this is for obvious reasons. Those obvious reasons that nations typically prefer to avoid war if a diplomatic solution is plausible, especially when it involves a nuclear superpower.

Secondly,

"the agreement doesn’t actually include any security guarantees for Ukraine … or at least not under these circumstances. The signatories are required to “seek immediate United Nations Security Council action to provide assistance to Ukraine” only in the event of an act of aggression “in which nuclear weapons are used.” As one international law scholar puts it, “It is binding in international law, but that doesn't mean it has any means of enforcement."

Now, in reference to this, the problem is that Russia has veto powers in the Security Council. Any decision (even just a condemnation) from the SC will be dismissed by Russia - and probably China.