To free traders, the new Nafta is a bitter pill to swallow. It introduces managed trade to autos, waters down the foreign rights of corporations and normalizes national security as a pretext for tariffs. Many of its improvements, such as on intellectual property and labor rights, were already in the 12-nation Trans-Pacific Partnership, from which President Donald Trump has withdrawn. But the verdict is different when judged by a different standard—how the world’s trading system survives the most protectionist U.S. administration in memory. The new deal shows the limits to Mr. Trump’s “America First” agenda and an underlying resilience to the existing order. The reason: the resistance Mr. Trump encountered from Congress, business, his own advisers and U.S. trading partners. This circumscribed his leverage and may again in the future. …Had U.S. negotiators proceeded without Canada, there was a real risk that Congress wouldn’t ratify an agreement. (full story with WSJ digital subscription only)