With the war in Ukraine raging and Russia’s Vladimir Putin threatening to use nuclear weapons should outside forces cross some undefined “red lines,” NATO’s Article 5 has understandably received greater attention, but not necessarily closer scrutiny. For those who have not studied Article 5, the impression, underscored by President Joe Biden’s repeated assurances, is that it is “ironclad.” It is not. Article 5 was, in essence, a successful bluff that was never called throughout the Cold War. Would NATO have gone to war if attacked by a nuclear superpower? Fortunately, we will never find out. Russia’s unprovoked invasion of Ukraine raises this question again unless President Biden’s commitment to Article 5 has removed any ambiguities and bluffs. The relevant parts of Article 5 are not definitive and state (underlining mine): “The Parties agree that an armed attack against one or more of them . . . shall be considered an attack against them all and …… each of them …..will assist the Party or Parties so attacked by taking …. such action as it deems necessary, including the use of armed force.” Once Article 5 is invoked, all NATO’s 30 governments must approve the use of force or a declaration of war. Once Article 5 is invoked, all NATO’s 30 governments must approve the use of force or a declaration of war. Article 5 was invoked only once on September 12, 2001, after the al Qaeda attacks on America. But not every NATO nation went to war in Afghanistan. Suppose, for example, a Russian cruise or ballistic missile attacked a resupply convoy inside Poland bound for Ukraine. Would that necessitate an Article 5 response? To be more provocative, suppose Russia employed a sub-kiloton nuclear weapon equivalent to 400 or 500 tons of high explosives—the largest conventional weapon the US has carried 10 tons—in a similar attack. Would that constitute Article 5? If Article 5 were invoked, would all NATO governments agree on using force or declaring war? If the answer were yes, what might be the appropriate responses? If consensus was not reached, how would that affect the alliance and the responses of those members willing to use force? Of course, these scenarios have been played countless times in countless war games in defence ministries, NATO staffs, war colleges and other venues. But those exercises were academic. Suppose NATO was forced to invoke Article 5 for the second time? In the United States, the Constitution raises an impossible contradiction. The president is commander-in-chief. Thus, he (or she) can legally order the military into action. But only Congress has the power to declare war. This contradiction was intensified by the Cold War. Had the Soviet Union launched its first nuclear strike against America, the commander-in-chief would have had only minutes to respond. If a counter-strike were ordered, a de facto war would have been declared without Congressional consent. Despite some near misses, this contradiction never arose. In these Ukraine scenarios with time for deliberations on Article 5, Congress would have to vote to use force or declare war. But suppose one or two members in the Senate chose to filibuster the resolution? Or sufficient members of both Houses, opposing war or fearful of the potentially existential consequences, failed to be present to deny a quorum. And worse, suppose the resolution was divided along party lines with the Vice President casting the deciding vote? With today’s septic politics, even a nightmare scenario would seem plausible. Of course, Congress could readily approve a resolution. But then what? Vladimir Putin must be aware of these political realities. If Putin thought he could bluff his way not only to deter but to shatter NATO cohesion through challenging Article 5 by threatening or launching an actual attack on NATO, would he do so? On the other hand, as with every Soviet leader during the Cold War, would the escalatory risks deter Putin? Joe Biden was criticized and even mocked for his ad-lib in last week’s Warsaw speech of “For God’s sake, this man cannot stay in power.” Biden and NATO have also declared that none of our forces will be sent to Ukraine. Nor, as Biden has reaffirmed, does he want to start World War III. However, his ironclad commitment to defending “every inch of NATO territory” must be taken very seriously. Any Russian attack on or against NATO is an entirely different proposition. Putin must know that President Biden has proven inflexible on several vital issues. Afghanistan is one. Biden said “out.” And the US departed leaving a country in chaos. Putin better understand that, according to the American President, Article 5 is ironclad. And that is no bluff. The writer is a senior advisor at Washington, DC’s Atlantic Council and a published author.