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.

The first president to ignore MacArthur’s warning was Harry Truman. In fact, MacArthur’s declaration that “there is no substitute for victory” contributed to Truman’s decision to fire him as supreme commander of UN forces in Korea in April 1951. MacArthur believed that the only way to win the conflict was to commit to complete defeat of the Chinese forces who had entered the war to support their North Korean allies. Truman worried that would trigger an all-out war with the Soviet Union (we now know from Soviet archives those fears was groundless, as MacArthur believed). Truman and his advisors insisted that stalemate and a divided Korea was the only realistic option.

Sixty-five years later, the result is a war that still hasn’t officially ended, despite a signed armistice in 1953. If MacArthur’s lead had been followed instead, there’d be no Kim Jong Un today to threaten his neighbors with nuclear annihilation—and possibly no Maoist regime to starve to death millions in the Great Leap Forward.

Another president, Lyndon Johnson, repeated the same mistake in Vietnam. That was a conflict specifically MacArthur warned against before his death in 1964, telling President Kennedy that, “anyone who embarks on a land war in Asia ought to have his head examined,” especially if there’s no clear-cut strategy for victory.

In fact, there was none. Instead, the United States sent more than half a million men to fight a limited war in South Vietnam while allowing a ruthless, determined enemy to maintain sanctuaries in Cambodia and Laos—and without unleashing the full weight of American air power. Vietnam ultimately cost America more than 53,000 lives and nearly broke America’s spirit, even as we ignominiously allowed the collapse of the South Vietnam we were pledged to defend—a debacle we’ve almost seen repeated in Iraq under Obama.

As for Iraq and Afghanistan, MacArthur would simply repeat his Congressional testimony in 1951: “You have got a war on your hands, and you can’t just say, ‘Let that war go on indefinitely while I prepare for some other war…’” Yet that’s precisely what both the Bush and Obama administrations did. And even while Obama still won’t take strong decisive action against ISIS, our forces engage have grown from a handful to more than 3000 personnel.

It’s so reminiscent of our initial involvement in Vietnam, it’s scary.

Sixty five years ago, Douglas MacArthur put his finger on the mistake that’s haunted us ever since: the failure to use military force without an equal commitment to final victory.

“When men become locked in battle,” he once told Congress, “there should be no artifice under the name of politics, which should handicap your own men, decrease their chances for winning, and increase their losses.” Yet that’s precisely what too many American presidents have done, bringing sorrow to tens of thousands of American families.

We’ve ignored MacArthur’s warning to great loss and peril. Now in the shadow of Orlando, Istanbul and Dhaka maybe it’s time we paid attention.