The US government blows things up because blowing things up is a component of GDP.  A rising GDP gives the government an excuse to claim that the economy is doing well, which gives them an excuse to borrow money to spend on blowing more things up, also known as "providing government services."  This borrowed money is provided by the central bank, known as the Federal Reserve, which prints money out of thin air to lend to the US government to blow things up, at interest.  This interest is also known as the national debt, and will never be repaid anyways so it doesn't matter.  Welcome to America.