February 2, 1848:
The Treaty of Guadalupe Hidalgo was signed or, as my friend likes to say, Chicanos were born

From the National Archives’ Prologue Magazine (Summer, 2008) article on the Disturnell Map of 1847 (above):

On February 2, 1848, a Treaty of Peace, Friendship, Limits, and Settlement was signed at Guadalupe Hidalgo, thus terminating the Mexican-American War. While the war was ostensibly about securing the boundary of the recently annexed state of Texas, it was clear from the outset that the U.S. goal was territorial expansion. Some decades earlier, the United States had secured the Louisiana Purchase, and President Polk now saw it as America’s “manifest destiny” to acquire access to a western ocean through the acquisition of Nuevo México and the Californias (which included parts of the present-day states of New Mexico, Arizona, California, Nevada, Utah, and Colorado). Ultimately, Mexico was obliged to cede Alta California, Nuevo México, and northern portions of the states of Sonora, Coahuila and Tamaulipas.

I’d write something more significant, but my mind is kinda racing with Lost theories.