Except the President cannot, alone, guarantee anything.
He is teh President of the united STATES. Going to war involves the STATES intimately, so to make a TREATY (which could guarantee something), the agreement of 2/3 of the Senate, which represents the STATES, is required.
Of course, the Sink Emperor never submitted a treaty or anything like one to the Senate for ratification on the Ukraine matter, so the TRUTH is - WE never promised them anything, and we don't owe them anything.
“Except the President cannot, alone, guarantee anything.”
Absolutely correct. British participation in World War I came about largely as the result of a lot of “executive agreements” over Belgium, etc.
The British people and Parliament were never asked their opinion (or consent) about going to war; they were just told that they were in it.