Mexico is entering a phase in which, at Trump's insistence, it will have to re-negotiate the North American Free Trade Agreement. That could imperil the export manufacturing sector that has become crucial to Mexico's economy since NAFTA took effect in 1994. But it also could let Mexicans discuss restrictions on billions of dollars in American farm exports, as well as measures to ensure to U.S. oil and gas, a topic that was kept out of the original trade pact when Mexico jealously guarded its oil.