What happens to international treaties?

These non-ratified treaties may sound like treaties and look like treaties, but they are nothing more than casual, perhaps symbolic, nonbinding agreements, because they are not valid expressions of constitutional doctrine

While Treaties are ratified by the Senate they are not American or even International law.  They are mere formal agreements (promises) and don't exist in perpetuity.  So breaking a treaty, when it no longer serves the interests of the country (or leader) is common amongst countries across the globe. To answer your question directly, President's have felt they can break treaties since the moment treaties were first created.  I'm no Trump fan, but this sort of thing isn't unique to him.

The current president thinks he has the power to do anything he wants. We will fix that in November.