Donald Trump is at a pivotal moment. He can heed General Mattis's warning -- delivered publicly, firmly, and respectfully -- or he can continue down his current, reckless path. This letter represents America's most-respected warrior telling the nation that he does not believe the president sees our enemies clearly, understands the importance of our alliances, or perceives the necessity of American leadership. We should be deeply troubled.

But this isn't just a pivotal moment for Trump. Republicans in Congress believed that General Mattis's appointment was one of Trump's best decisions as president, and Mattis's very presence at the Pentagon reassured the party and (more importantly) the public that an inexperienced, impulsive, president would listen to wise counsel. After reading this letter, will Republicans in Congress retain their faith in Trump's judgment? Will they continue to view him as the leader of the GOP, the man they defer to in politics and policy?

Now is the time for Republicans in Congress to declare their independence from the Republican in the White House and refuse once and for all to rubber-stamp Donald Trump's whims and desires.