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.

He has all but abandoned Afghanistan to the Taliban.

He also didn’t just ignore MacArthur’s warning but mocked it, by throwing away America’s hard-won victory in Iraq.  By pulling out all remaining US troops, he opened the door to ISIS’s explosive upsurge and the atrocities of its murderous followers, including now in this country.

Yet Obama hasn’t been alone in ignoring MacArthur’s warning.

From Korea to Iraq and Afghanistan, one administration after another has repeatedly pursued the opposite course, with a cost in American lives, treasure, and prestige that’s almost incalculable.