Criticized over his withdrawal of United States military forces from Syria, President Trump says he can still prevent a Turkish onslaught against innocent Kurds.

"As I have stated strongly before, and just to reiterate," Trump tweeted on Monday, "If Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the economy of Turkey (I’ve done before!)."

Those words, oozing as they are with hyperbole, would have been credible one year ago. Today? Not so much, because Trump has spent the past year blowing up his red lines.

[Read: 'Stain on America's honor': Trump gives Turkey green light to attack US-Kurdish allies in northern Syria]

Consider a few examples. Facing mounting Cuban-enabled suffering in Venezuela, Trump in April warned Havana that unless its security forces were "immediately" withdrawn, it would face crippling sanctions. Six months later, those forces remain entrenched in Venezuela and Trump's crippling sanctions remain nonexistent.

Take another example closer to Turkey. Trump has consistently warned Syrian dictator Bashar Assad that his use of chemical weapons is intolerable and will meet consistent U.S. retaliation. But where Trump once matched those words with action, he is now silent. The U.S. government confirmed last week that Assad used chemical weapons again in May. Trump has responded with silence.

Same too with North Korea where, even as Trump pledges his unconventional diplomacy will produce outsize results, Pyongyang is testing new nuclear launch capabilities and advancing its ballistic missile program. Trump's policy now looks like simple appeasement.

This is a credibility gap that reverberates across broader American foreign policy. It also diminishes Trump's credibility on others issues, such as Iran, where his policy decisions have otherwise been correct. Where Trump gives ground and then makes threats that are unbound from credibility, the Iranian hard-liners find new reason to test him. It also ignores the political fact that once Erdogan is committed to an offensive in Syria, he will have very limited political room to end it under American pressure. Once engaged, it will be a nationalist adventure that galvanizes Erdogan's base.

In short, Trump has degraded the deterrent quality of his tweets and of U.S. foreign policy. Establishing red lines he then fails to enforce, Trump's foreign policy is closer to his predecessor's than he might care to admit.