And he says the fight over the Strait of Hormuz is where it may finally break.
Most people look at the Strait of Hormuz and see another dispute over shipping, but Admiral Montgomery sees something far more dangerous.
For almost 80 years, the U.S has enforced one simple principle: the world's most important sea lanes stay open, nobody gets to charge a toll by force, and no country gets to rewrite international maritime law with a gun to people's heads.
Iran is now trying to do exactly that.
What's wild is that Oman isn't dismissing the idea outright; it's prepared to discuss it, and that should make everyone pay attention.
Because if Iran succeeds in turning the Strait of Hormuz into something it controls rather than something the world uses freely, the consequences stretch far beyond the Middle East.
Today it's Hormuz, tomorrow another country tests the rules somewhere else.
Admiral Montgomery's argument is that once America accepts one exception, the entire system it spent generations building starts becoming negotiable.
But here's the dilemma he kept coming back to: America still has the military power to defend that system, but the question is whether it can keep paying the price.
He openly admits decades of continuous deployments have strained the Navy, worn down aircraft, stretched destroyers, and tied up forces that Washington increasingly needs in the Indo-Pacific to deter China.
So America finds itself caught in a trap of its own making.
It can't easily walk away without inviting challenges to the international order.
But it can't stay everywhere forever without exhausting the very military that underpins that order.
@MarkCMontgomery
