They settled in Texas, and claimed the land even though it belonged to Mexico at the time, against the wishes of the US Government. Then they couldn't hold out forever against the Mexicans. Their currency was worth kindling. So they joined the US to get US military protection. The US defeated the Mexicans and got the area now known as New Mexico, Arizona, California, Nevada, Utah, and Colorado if I recall correctly (it's been like 35 years since I studied this stuff).
These were rogue settlers from the United States who decided to claim land that wasn't theirs. They get their patooties in a sling. The United States accepts them, and wins the war against Mexico. But no good deed goes unpunished. They turn around and join the Confederacy in the Civil War.