The Filibuster Movement, or filibusterism, of the 19th century was an outgrowth of the idea of Manifest Destiny. There were many who believed that Americans were destined by their superiority to expand across the continent, or even further, to bring civilization to the New World. Filibusters were men who dared to pursue this by invading other nations with their own private forces independent of the US government. The most noted among them was William Walker. In 1853, Walker decided to claim some land in Mexico.
Now, invading a country with whom the U.S. was at peace may seem like it would be breaking laws, and that would be correct. It was a direct violation of the Neutrality Act. This did not stop Walker, though, who recruited a small group of adventure seekers (who were really just looking for opportunities after failing to get rich from the Gold Rush) and found a ship to sail from the Bay Area to Baja California in Mexico. His original ship was taken by the American military, which was cracking down on filibusters, but the ambitious Walker simply found another and snuck away with a makeshift army of fewer than 50 men.
On November 3, 1853, after landing in Baja California, Walker captured the state capital of La Paz and proclaimed the region the Republic of Lower California. The new country, which was never recognized as a country, needed a leader, and Walker was, naturally, made the president. This invasion may have been viewed as highly illegal by both American and Mexican authorities, but the American public loved it. Filibusters were cool. They embodied Manifest Destiny in a way that no one else did. Because of this support, people actually traveled to Mexico to join Walker’s territory.
Walker's adventures in Mexico ended when he overstepped his bounds, but he had more luck in Nicaragua, where he was recognized as president for a while. He also invaded Honduras. Read the story of William Walker and his temporary conquests at Cracked.
Image: The Costa Rica National Monument depicts five nations chasing William Walker out of Central America. Photo by Mariordo (Mario Roberto Durán Ortiz).