> If a nation ignores a treaty, the reaction should not be "Oh, that was to be expected".

A nation can withdraw from a treaty instead of ignoring it. The US just withdrew from the INF treaty. In retaliation, Russia withdrew too (they were the only signatories).

In 1936 Japan withdrew from the Washington Naval Treaty and built the formidable battleship Yamato (armed with 18 inch guns, by far the largest in WW2). What was the world supposed to do? The US imposed various sanctions on Japan. At some point the sanctions became so hard that they amounted to an economic death sentence. WW2 was not averted.

The unpleasant truth is that international treaties are not worth a whole lot. For the simple reason that there's no international equivalent of law enforcement.