MANY PEOPLE who viewed Donald Trump as unprepared and temperamentally ill-suited to be president initially reassured themselves that a sober-minded Republican majority in Congress would restrain him. Speaker Paul D. Ryan’s (R-Wis.) pitiable farewell this week spotlighted the emptiness of those hopes. Mr. Ryan allowed the president and his allies to corrupt the House Intelligence Committee and treat federal law enforcement as an enemy. Like Senate Majority Leader Mitch McConnell (R-Ky.) and most other ostensible GOP leaders, Mr. Ryan abetted the president as he sought, fruitlessly, to use fear and bigotry to preserve the House Republican majority in the 2018 elections. Fittingly, Mr. Ryan’s final act this week was to indulge the president’s capricious decision to shut down the government rather than accept a budget compromise.

The second line of defense was going to be the “grown-ups” in the administration: Secretary of State Rex Tillerson, Defense Secretary Jim Mattis, national security adviser H.R. McMaster, economic adviser Gary Cohn, U.N. Ambassador Nikki Haley, intelligence chief Daniel Coats. These are people who understand the value of alliances, the risks of trade wars, the even more unspeakable risks of real wars. Over the past two years, they did at times restrain the president’s worst impulses — in ways we know and likely in ways we have yet to learn.

But with Mr. Mattis’s announcement this week that he will step down in February, almost all the “grown-ups” will be gone. More to the point, the stimulus for the defense chief’s decision — Mr. Trump’s impetuous decision to withdraw all U.S. troops from Syria and many from Afghanistan — seems to show that the president believes he no longer needs advice, if he ever did. “I watch the shows,” he once said when asked where he turns for military counsel. The country seems now at the mercy of Mr. Trump and whatever he has heard most recently on Fox News.

The peril does not stem primarily from Mr. Trump’s wrongheadedness on issues, though we do disagree with him on most things. We believe the sudden withdrawal from Syria will embolden Iran, delight Russia, allow the Islamic State to spring back, endanger tens of thousands of civilians and teach other opponents of Islamist terrorism never again to trust the United States. But counterarguments could be made: that in Syria, U.S. troops are being put in harm’s way without a clear mission; that 17 years is long enough in Afghanistan; and so on.

But Mr. Trump initiated no process to weigh those arguments before astonishing allies, Congress and his own staff with his flip-flop. He apparently did not consider that he would sabotage his own ambassador’s negotiations with the Taliban, which, for the first time, seemed promising. Put simply, he still does not know, and clearly does not care to learn, how the government of a major power should operate.

So what now? A third line of defense was hoped to be the other institutions of democracy — the courts, media, civil society more broadly — and norms built up over decades and centuries. In some heartening ways, they have held. But when the president can install an aggressively unqualified yes-man as acting attorney general, and when that appointee can simply ignore qualified ethics advice, it becomes clear how tenuous these norms can be.

Which brings us back to the first line of defense. Congress is the Article 1 power for a reason. The Constitution grants it awesome powers over war and peace, appointments, budgeting, trade and, in extreme circumstances, impeachment. By practice it has ceded much of its authority in recent years, and its muscles have gone flabby. Given the dangers ahead, the country’s only hope may be for Congress, in a responsible but determined way, to begin exercising those muscles again.

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