Mother Jones' Kevin Drum puts it simply:

We've spent the past two decades militarizing our police forces to respond to problems that never materialized.

The two problems were the drug-fueled crime wave of the '70s and '80s and the post-9/11 fear that local police forces would soon be overwhelmed with local terrorist plots. In America, big problems require big guns, and so one response to these fears was the so-called "1033 program", where the Department of Defense distributed surplus military equipment to local police forces. As Amanda Taub explains:

The 1033 program's roots lie in the drug war — hence the counter-narcotics impetus. It was originally created in 1990, as part of the National Defense Authorization Act, which authorized the Pentagon to transfer military equipment to local law enforcement if it was "suitable for use in counter-drug activities." In the wake of the September 11th attacks, the program's focus has expanded to include counter-terrorism activities as well. While the 1033 program's intent may have been to equip specialized units for extreme, dangerous situations, fighting al-Qaeda sleeper cells or powerful drug cartels, the effect has been to incorporate SWAT-style raids into ordinary police operations. That includes, but is certainly not limited to, the serving of search warrants. This may partly be because the program requires that all equipment issued through the 1033 program be used within one year of the date it is granted. That means that if police departments want to keep their new gear, they can't wait for a rare emergency like an active shooter or hostage situation in order to use it.

Read those last few sentences carefully. Police get all this equipment and, as a condition of the program, need to use it within a year. What they don't get is training. The ACLU's Kara Dansky, who authored an important report on police militarization, told Vox she was "not aware of any training that the government provides in terms of use of the equipment," or of "any oversight in terms of safeguards regarding the use of the equipment by the Defense Department."

So police have all this military equipment, very little training on how to use it, and a requirement that they deploy it within a year. But the problems they were supposed to use the equipment against have either eased or vanished.

The crime wave that ripped through the country in the '70s and '80s broke in the '90s and continued to fall through the Aughts. There were 23,000 murders in 1980. There were 14,827 in 2012. (Note that America's population grew by more than 80 million people during that time.) Meanwhile, al Qaeda never became a major problem for local law enforcement.

The result is that the equipment gets used — and used badly — to put down mostly peaceful protests in places like Ferguson, or to raid organic farms. And it can mean that communities come to view their militarized police forces as a threat:

It became so clear the awfulness of the situation. Communities need police. But here & now, the slightest police presence enrages people. — Antonio French (@AntonioFrench) August 16, 2014

"Police militarization was a mistake," concludes Drum. "You can argue that perhaps we didn't know that at the time. No one knew in 1990 that crime was about to begin a dramatic long-term decline, and no one knew in 2001 that domestic terrorism would never become a serious threat. But we know now. There's no longer even a thin excuse for arming our police forces this way."