In any time and place, war is fiendishly simple. It is the ultimate zero-sum contest — you win or you lose.

That eternal truth is so obvious that it should not need to be said. Yet even after the horrific slaughter in Paris, there remains a distressing doubt about whether America’s commander in chief gets it.

President Obama has spent the last seven years trying to avoid the world as it is. He has put his intellect and rhetorical skills into the dishonorable service of assigning blame and fudging failure. If nuances were bombs, the Islamic State would have been destroyed years ago.

He refuses to say “Islamic terrorism,” as if that would offend the peaceful Muslims who make up the vast bulk of victims. He rejects the word “war,” even as jihadists carry out bloodthirsty attacks against Americans and innocent peoples around the world.

He shuns the mantle of global leadership that comes with the Oval Office, with an aide advancing the preposterous concept that Obama is “leading from behind.” He snubs important partners like Egypt, showers concessions on the apocalyptic mullahs of Iran, and called the Islamic State the “jayvee team” even as it was beginning to create a caliphate.

Having long ago identified American power as a problem, he continues to slash the military as the enemy expands its reach. In a globalized era, the Obama doctrine smacks of cowardly retreat and fanciful isolation.

In an accident of timing that captures his cluelessness, the president actually declared on Friday morning that the Islamic State had been “contained,” practically boasting in a TV interview that “They have not gained ground in Iraq and in Syria.”

What gall. What folly.

Paris is the final straw. Obama’s exemption from reality has expired. He must either commit to leading the free world to victory, or step aside so someone else can.

There is no more time to avoid the truth of war. America must organize the combined forces of the civilized world before the Islamic State makes good on its vow to “taste” more American blood.

If Obama cannot rise to the challenge of leadership in this historic crisis, then, for the good of humanity, he should resign.

As a top intelligence adviser told me yesterday, “What they did in Paris means they are coming here.”

In fact, they already are here. Law enforcement officials say the FBI has as many as 1,000 investigations open into Islamic State sympathizers inside the US.

Is America ready to stop multiple assault teams of suicide bombers? Is New York ready? Or Chicago, Los Angeles or Washington, DC?

Because Paris was a grand success to the terrorists, the propaganda value acts as an incentive for attacks on other Western cities. While sparing no effort to stop them here, we must simultaneously destroy them in their foreign bases.

World War III began when Osama bin Laden declared war on the United States, though we did not grasp the significance until 9/11. The collapse of the Twin Towers, a smoking hole in the Pentagon and a downed jetliner in Pennsylvania revealed the price of our inaction.

The single greatest attack ever against America galvanized the nation and defined a new generation of policymakers and warriors.

Yet Obama always remained curiously cool about the whole endeavor, denouncing the invasion of Iraq as dumb while holding up Afghanistan as a necessary war. Once he got to the White House, though, he showed no conviction about Afghanistan either, surging troops only to demand that they return home quickly.

The pattern has never changed, and his relationship with a rotating cast of military leaders remains rocky. Robert Gates, secretary of defense under both President Bush and Obama, said in his memoir that Obama’s distrust of the military was destructive of the very mission he had given the troops.

After a heated 2011 meeting on Afghanistan, Gates concluded that Obama “doesn’t believe in his own strategy, and doesn’t consider the war to be his. For him, it’s all about getting out.”

Another former military leader, Gen. Jack Keane, notes that Obama never once agreed to the full request of his commanders. If they ask for 10,000 troops, Obama agrees to 5,000, 3,000 or none.

The raid that got bin Laden marked the high point of Obama’s commitment. He turned that achievement into political gold in 2012 and declared the “tide of war is receding” to justify his decision to withdraw from the field of battle.

It was a convenient figment of self-interest, as if his wish would make it true. Instead, the strategic dominos fell quickly as war metastasized. The hard-won gains in Iraq were reversed, Syria descended into hell and the Islamic State was born in the vacuum.

Its ruthlessness and success in capturing territory enabled it to supplant al Qaeda as the most dangerous terrorist network. It has become the proverbial “strong horse,” with each terrifying attack bringing more recruits and more financing.

In the last two weeks, it shot down a commercial Russian airliner over Egypt and carried out bombings in Beirut. And then came Paris.

Its ability to inflict unprecedented casualties in such far-flung locations marks a growing strength and sophistication. The terrorists smell weakness and have increased the pace of their aggressive expansion. Their aim of global conquest must be taken seriously.

French President François Hollande understands the meaning of Friday’s slaughter. He called it an “act of war” and vowed that “France will not show any pity” against those who carried out the barbaric acts. World leaders quickly expressed their condolences and condemnation.

Yet it remains doubtful if our side is truly committed to winning. The determination and unity the free world showed after 9/11 faded as casualties, mistakes and politics eroded the mission.

So we are back to square one again, facing a stronger and more emboldened enemy. The time has run out for half measures and kicking the can down the road. The enemy must be destroyed on the battlefield before there can be any hope of peace.

If Obama cannot rise to the challenge of leadership in this historic crisis, then, for the good of humanity, he should resign. Those are the only options and it is his duty to decide.