Like all Americans, my wife and I watched with horror as the news came about the mass shooting in Orlando by an ISIS-inspired fanatic.  It made me realize how Douglas MacArthur, one of America’s greatest military leaders and the subject of the biography I just published, is more relevant than ever for understanding our age.

The massacre in Orlando and the attacks this week in Istanbul and Dhaka demonstrate what happens when we disregard the advice MacArthur gave us more than sixty-five years ago:  “There is no substitute for victory.” It reminds us of what happens when the United States fails to use all its means to defeat a vicious and committed enemy, in this case ISIS.

Indeed, Douglas MacArthur taught the single important strategic lesson any American president can learn: before starting a war, make sure you are committed to winning it.

That’s a lesson President Obama has repeatedly ignored.  He has refused to fully commit U.S. strength to crushing ISIS’s enclaves in Iraq and Libya, and to severing their lines of recruitment and support.