An organized, heavily armed group took over an empty federal building on Saturday in a small city in Oregon, where they've been camped out ever since.

The armed occupiers, who are now hunkered down at the Malheur National Wildlife Refuge headquarters near Burns, Oregon, broke off from a larger group of protesters who were demonstrating against the imminent imprisonment of two Oregon ranchers convicted of arson.

But these occupiers — who have also been called militants, extremists, freedom fighters and terrorists — are not the first armed group we've seen do this over the past few years. In fact, the number of so-called militias in the U.S. shot up after the 2008 election of President Barack Obama. The question is, why?

The 1993 standoff

Fire engulfs the Branch Davidian compound near Waco, Texas, on April 19, 1993. Image: Ron Heflin/Associated Press

If we're trying to pinpoint the beginning of the America's modern militia movement, one event that arguably generated the most attention happened in Waco, Texas, in 1993.

Federal agents raided the compound of a religious group whose leader was accused of stockpiling illegal weapons. Four ATF agents and five members of the group were killed in a shootout.

But that was just the beginning of a standoff that started on Feb. 28 and ended on April 19, 1993. That day, tanks poked holes in a compound where the leader, David Koresh, was living with dozens of followers. The FBI filled the building with tear gas, but a fire started and killed around 80 people, including Koresh. Many right-wing fringe groups declared them heroes who died in the defense of gun rights.

For those who already sympathized with the idea of militias, the catastrophe in Waco showed the government would kill to get its way. "More than any other event, the debacle ignites the militia movement," according to the Southern Poverty Law Center (SPLC).

What sparked the resurgence after 2008

Supporters of rancher Cliven Bundy gather on I-15 just south of Mesquite to demand the release of his impounded cattle on April 12, 2014. Image: Jason Bean/Las Vegas Review-Journal/Associated Press

The militia bedrock established in the 1990s eroded going into the new century, but the sentiment never quite disappeared.

Anti-government "patriot" groups and militias soared again following Obama's election win in 2008.

Image: SPLC

The number of anti-government groups climbed from 149 to 512 between 2008 and 2009, and peaked at 1,360 in 2012. In 2014, the latest year for which the SPLC has data, 874 anti-government groups were still in existence, 23% of which qualified as militias — anti-government groups that undergo weapons training.

Long-held fears of the federal government merged with fear of an African-American president, possible gun control legislation and other far right-wing, anti-government concerns to reenergize militias.

The movement of the 1990s was more connected and more violent than the largely disparate groups of today, according to the SPLC. But today's militias are more galvanized by racism that stems from Obama's election and the increasing presence of Latinos in the U.S.

What do they want?

Arden Bundy, son of rancher Cliven Bundy, flies the American flag under the I-15 highway near Bunkerville, on April 12. Image: Jason Bean/Las Vegas Review-Journal/Associated Press

Woven throughout militia ideology is the supremacy of local rights over the rights of the federal government. Many of them hold county government in higher esteem than state or national government.

And their suspicion of the federal government is visceral. The militants in Oregon are demanding that the federal government give up Malheur National Forest because they believe the federal government has no right to the land.

That's the same reason these group members value their guns. Armed citizens, in their view, are the only people who can prevent the federal government from imposing its will on wary civilians. When these groups ban together, it's usually because they feel the government has gone too far.