Emmy-winning presidential historian Michael Beschloss is the author of nine books on the American presidency, and his latest, out Tuesday after a decade of work on the project, zeroes in on one particular part of that complicated job: being Commander in Chief.

Presidents of War examines how U.S. presidents have — successfully and unsuccessfully — led America to war, during the War of 1812, the Spanish-American War, the Mexican-American War, the two World Wars, the Korean War and the Vietnam War.

He spoke to TIME about the book, how the American approach to war has changed and why President Trump might want to brush up on some history.

What’s in here that people won’t find in other books on presidential history?

What the book is really about is that the founders of this country were terrified of one particular thing, and that was that Presidents of the United States would be able to start wars almost single-handedly. They looked at England and European monarchs, and one of the things they wanted most to avoid was, when the monarchs grew unpopular, they would start a war as a pretext to unite the country and get more popular — even if the war wasn’t necessary. As a result, you look at the Constitution and it says if you want a war, Congress has to give a war declaration.

Nowadays presidents can start wars almost single-handedly. Last time we had a war declaration from Congress was 1942, and we’ve had some pretty major wars since then. If you go through the history of presidents waging major wars, over 200 years, there have been a lot of cases where presidents have used those wars to expand their own power, to trample civil liberties, to constrict our democracy and, in certain cases, almost risk dictatorship. And all those things are worries that we’ve got today. ...