Several recent events have many Americans wondering what the government could be preparing for; and with the current status of global politics and economics, the possibilities are quite scary.
Many Americans were startled to see reports on Monday that the Department of Homeland Security was purchasing 2700 armored vehicles, all of which are fully equipped with armor strong enough to withstand ballistic arms fire, mine blasts, and IEDs, and are to be used domestically. But this massive purchase of military-style vehicles is just the tip of the iceberg for several Americans who have been closely following the U.S. government's recent activity.
In March, 2012, it was reported that the DHS was purchasing 450 million rounds of .40 caliber ammunition. It was then reported later in the year that they were purchasing 200 million rounds of .223 and .388 hollow point. They then added another 200,000 rounds in January, 2013 and 21.6 million in February. 
Aside from massive ammunition purchases and rolling out domestic tanks, the government has also demonstrated its preparedness for a potential crisis by establishing "Emergency Centers." In 2009, the 111th Congress released H.R. 645, which authorized the creation of FEMA emergency camp facilities which would house U.S. citizens during a national emergency or crisis. 
As for why the government is preparing in such a way, that can be left to opinion, analysis, speculation, and theory. The government has become more serious on controversial issues such as gun control, which has many people people fearing civil unrest and U.S. banks have been told to make plans for preventing a potential economic collapse, fueling the idea that our economy is not very strong. As Alex Allen and Liberty Battles reported earlier this year, there are several reasons as to why people believe tyranny may arise in America, including urban warfare drills and military documents outlining plans for civil unrest. Could all of these thoughts floating around be the U.S. government's justification for preparing for something big as it appears they are doing? Only time will tell.
Sidenotes: Most of the ammunition purchased by Homeland Security is hollowpoint ammunition, which has been outlawed for use in international warfare and could only be used domestically. Article: New Legislation Authorized FEMA Camps in U.S.