The Mexican-American War of 2018.
"Following the storming of the United States' southern border by Mexican migrants acting at the behest of the Mexican President, the US declared war, expelling 20 million Mexicans illegally residing in the US and forcing Mexico to cede the Baja Peninsula and Yucatan to the US, which became home to the the Trump Grand Cabo Resort and Trump Playa del Carmen Resort."
Ah yes. The Duchy of Grand Fenwick gambit.