Now, the following isn't a joke. But it is true, and kinda funny.
A few years back, my girlfriend and I visited San Antonio and took a tour of the Alamo. I didn't know much about the history of the region and it was really interesting to hear the story of the formation of the state - particularly the details leading up the the battle of the Alamo.
The tour guide told us the story: After Mexico won its independence from Spain, in 1821, they were eager to develop the wilderness region of Texas and passed a law encouraging settlement of the area. They welcomed anyone willing to work the land (and defend it from Indian attacks) to come and make land claims. They even advertised in US newspapers encouraging immigration to the area.
But soon thereafter, a more conservative administration came to power in Mexico City and radically changed the immigration policy. In part because many of the US immigrants owned slaves and were ignoring Mexico's new law outlawing slavery, and in part because they feared Americans would threaten Mexican sovereignty in the region, they outlawed all future immigration of US citizens.
With a suspicious grin, the tour guide continued: He pointed out that when you have a land ripe with opportunity on one side of a border, and throngs of people eager to take advantage of that opportunity on the other, keeping them out isn't as easy as just passing a law. US immigrants continued to pour into the region, ignoring the Mexican immigration laws. Rather than welcome their new 'guests', the Mexican Army was ordered to do whatever it could to keep them out.
The rest is 'history', as they say. I guess we can chalk it up to "what comes around goes around, eh?"