He campaigned for president as an unabashed imperialist. Shortly after taking office, he ordered the U.S. Army to engage in acts that were designed to provoke a war, which they did. The ensuing conflict was unpopular, but thanks to brilliant leadership by generals who belonged to the other party, it was wildly successful.
He sent a diplomat to negotiate a peace treaty based on substantial territorial concessions. He subsequently decided that his terms were too lenient, but his representative ignored his instructions and made a deal based on his initial instructions. He grudgingly accepted that deal, and America became a much larger country.
Other than the part about the brilliant military leadership, it sounds a bit like Trump, but it is actually James K. Polk. I have included it in my list of underappreciated turning points because it made the Civil War much more likely. Prior to the Mexican War, moderates of both parties had more or less reached a detente on the issue based on the Missouri Compromise, but the new annexation put slavery in the territories squarely back on the table, and this time, the various efforts to solve the problem (the Compromise of 1850, popular sovereignty, and Dred Scott) were totally unsuccessful. The center could not hold. A dramatic change to the nation was in the air.