When the artful prime minister of Britain, Harold Macmillan, was asked what he considered the greatest threat a statesman might face, he replied, “Events, dear boy, events.” Those running to become president might respond with the same answer, for in an election “events” can also transform the political landscape overnight.

The Russian invasion of Georgia is such an event. It hardly came out of the blue, for the oil rich nation has been a thorn in Russia’s side ever since it declared independence from Moscow in 1991. The land of Stalin’s birth and home to the final foreign minister of the Soviet Union, Eduard Shevardnadze, has rarely enjoyed freedom from Russia, except briefly for three years after the Bolshevik Revolution, and again 70 years later when the Soviet Union crumbled.

The timing of Vladimir Putin’s move may have been a surprise, taking advantage of President Bush’s absence from America when he was attending the Games in Beijing, but a military incursion has long been in the cards. It was just such a test of battle readiness that John McCain has long dreamed of, and that Hillary Clinton alluded to in her “3 a.m. telephone call” commercials questioning the preparedness of Barack Obama.

When news came that the Russians were bombing Georgia, Mr. McCain was fast out of the gate, swiftly condemning Mr. Putin for his bellicosity, demanding an immediate cease fire, urging the E.U. to censure the Russians, and calling a summit not only of the United Nations Security Council, enfeebled by Mr. Putin’s veto, but of the G-7’s foreign ministers. He called for a neutral peace-keeping force to drive the Russians from Georgia.

The invasion provided Mr. McCain with a perfect opportunity to display his suitability to be commander in chief and a chance to explain why he believes the G-7 should never have blessed Russia with a place at the table. At the same time he showed how as president he would operate foreign policy through a “League of Democracies.”

By contrast, Mr. Obama was slow off the mark. He began by gently urging peace through diplomacy, then took a full day before taking Mr. Putin to task and calling for the U.N. to intervene. Mr. McCain looked confident, presidential, and in command, while Mr. Obama, caught enjoying a beach holiday in Hawaii, seemed ill prepared and hesitant. Round one of the Georgia crisis goes to Mr. McCain.

So far it has suited both candidates to stress their differences in approach to handling troublesome foreigners. Mr. McCain, a Vietnam veteran and prisoner of war, the ranking member of the Senate Armed Services Committee, sharp critic of the Bush administration’s handling of the occupation of Iraq, and chief architect and champion of General Petraeus’s successful surge, has presented himself as a willing warrior if circumstances demand that America push back against totalitarian bullies.

Mr. Obama, meanwhile, in order to win the Democratic nomination, has promoted himself as the peace loving candidate, ever eager for jaw-jaw rather than war-war, even promising to spend his first year in office visiting the world’s despots to see whether he can persuade them to abandon their wicked ways. Since Mrs. Clinton has been dispatched, Mr. Obama has toughened up somewhat, yet he has chosen to paint Mr. McCain as a reckless warmonger who would happily billet American troops in Iraq for the next 100 years.

Mr. Obama has always insisted he is not a pacifist. “I don’t oppose all wars. What I am opposed to is a dumb war. What I am opposed to is a rash war,” he said. Put like that, few would disagree. But how far he is willing to allow, say, Iran’s nuclear weapons program to progress before mounting an air strike is anyone’s guess. How far he will back Ukraine when it, too, comes under Russian pressure to buckle to its demands, as it surely will, is also far from clear.

But who would make the most effective commander in chief is not solely a matter of which candidate is the best leader in a time of crisis. It is a fundamental question Americans must ask of themselves before casting their vote in November.

Are we, as we were for more than two decades after the end of the Vietnam War, suffering so severely from war fatigue that we are inhibited from taking military action when we think it is deserved? Has our experience in Iraq these last five years been so traumatic that the next president must provide a period of rest and recuperation while we mull our international responsibilities?

Are we looking for a president who will boldly lead us when, as in Georgia this week, we are confronted by a mighty power bent on swallowing wholesale a small and defenseless ally? Or do we think we can dispense with our global duties as soon as we are able to withdraw our troops from Iraq and Afghanistan and then watch the world fall apart from the distance of our own shores?

In fact, there is no such choice. The world is shrinking so fast that if you listen carefully you can hear its contractions. Our prosperity depends upon trade, making us inextricably linked to all other nations. Their problems are ours, whether we prefer to believe it or not. And the abiding lesson of September 11, 2001, is that America can no longer take shelter behind its oceans. If we do not help others combat tyrants, we shall soon find them knocking at our own door.

nwapshott@nysun.com