DOD 1033 is the Federal program that is primarily the reason why you see police departments across the country looking like soldiers. Basically the program gives local police departments military equipment FOR FREE.
The Missouri Department of Public Safety states:
"The Department of Defense Excess Property Program (1033 Program) is authorized under federal law and managed through the Defense Logistics Agency's Law Enforcement Support Office (LESO) in Ft. Belvoir, VA. The 1033 Program provides surplus DoD military equipment to state and local civilian law enforcement agencies for use in counter-narcotics and counter-terrorism operations, and to enhance officer safety. The Missouri Department of Public Safety is the sponsoring state agency responsible for administration of the 1033 Program in Missouri."
In 1990, congress passed the National Defense Authorization Act. Originally section 1208, in 1996, the part of the Act that sets up this program was changed to section 1033.
In a good article about this, Newsweek writes: "The idea was that if the U.S. wanted its police to act like drug warriors, it should equip them like warriors, which it has—to the tune of around $4.3 billion in equipment, according to a report by the American Civil Liberties Union. The St. Louis County Police Department’s annual budget is around $160 million. By providing law enforcement agencies with surplus military equipment free of charge, the NDAA encourages police to employ military weapons and military tactics."
"St. Louis County law enforcement agencies have, through the 1033 Program, acquired the following “tactical” equipment, according to Mike O’Connell, Communications Director for the Missouri Department of Public Safety: https://docs.google.com/spreadsheets/d/1zTFBan2ZjyLrK05yW5OtSIhr9w3SsWNhUnl_fzZzy6o/pubhtml?widget=true&headers=false " [Newsweek]
FAQ about the program on dispositionservices.dla.mil
Earlier AP report on the program AP IMPACT: LITTLE RESTRAINT IN MILITARY GIVEAWAYS
Article about 2012 pause in the program Pentagon halts program supplying police agencies
Here's another article about the program & about St. Louis's participation in it.