It's easy to break things, and hard -- sometimes impossible -- to fix them. A handful of poor decisions by the captain and bridge crew of the Titanic sent that ship to the bottom. Four years of a hypothetical Trump presidency 'checked' only by a feckless Republican Congress would be a strong incentive for many U.S. allies to make new arrangements minimizing their exposure and risk and marginalizing a newly dangerous and unreliable U.S. This would be hard for a future president to undo; it would certainly not be easy.

Fortunately the odds of this actually happening are low and getting lower.