I highly recommend Rachel Maddow's book "Drift - The Unmooring of American Military Power". Maddow points out that the framers of the US Constitution were well aware of the historical tendency for rulers to go to war, so they deliberately made it difficult for the President to go to war by lodging the war-making power in Congress, where open debate and cooler heads might prevail. She argues that in recent times Presidents have arrogated war-making powers to themselves, and Congress has let them, with disastrous results. (think of Obama's Libyan bombing, for which he argued he needed no Congressional approval). This is not a Republican or a Democratic thing - presidents of both parties have played this game in recent years, including Clinton and both Bushes as well as Obama.
This book will not be a comfortable read. You will no doubt be appalled at how we have moved war - real destructive killing war - underground with contractors so that the American people hardly know (or care) that we are at war. We now kill people in foreign lands from drones without trial, without a declaration of war by Congress, and without any serious oversight by any elected official. If you are uncomfortable with this, you ought to be. I certainly am.