I do believe that many of the wars undertaken by the United States are done in the name of democracy and even capitalism. Not always though. And even in the cases where I believe this is the truth, that doesn't mean it's right. You can't really spread democracy by pointing guns in people's faces.
Wars undertaken by the United States are always done in the name of democracy. For example, the war in Iraq was undertaken to fight possible weapons of mass destruction. The Civil War, in addition, was done to abolish slavery and have a uniform system of taxation that would benefit the country as a whole.
Wars undertaken by the United States are not done in the name of democracy. Wars started by the United States in the 21st century lacked popular support among democracies, as well as the countries which were invaded. In addition, the United States has propped up dictatorships and does not support democratically elected governments which hold opposing views.
As an American citizen I can not say that the wars we undertake are done in the name of democracy. I can't really say why we have gone to war in the last few campaigns we've entered. If I had to pinpoint reasons, I would say the last two were the result of a corrupt administration. They had nothing to do with democracy, they were all about power.