LATVIA, ESTONIA, and Lithuania form the Achilles’ heel of the NATO alliance. They are protected by its Article 5 guarantee that an attack upon one will be regarded as an attack upon all. Thus, the United States has an unambiguous and undeniable responsibility to deter and defend attacks on the Baltic states. Given their size, proximity to Russia and substantial Russian-speaking minorities, this is a daunting requirement. It is not difficult to imagine scenarios in which either U.S. or Russian action could set in motion a chain of events at the end of which American and Russian troops would be killing each other.

There is currently a lively discussion among Russian hard-liners about how Russian dominance in both conventional forces and tactical nuclear weapons in Central and Eastern Europe could be used to Russia’s advantage. Putin has talked publicly about his willingness to use nuclear weapons to repel any effort to retake Crimea—noting that he relied on Russia’s nuclear arsenal during the Crimean operation. In these debates, many ask whether President Obama would risk losing Chicago, New York and Washington to protect Riga, Tallinn and Vilnius. It is a troubling question. If you want to either dumbfound or silence a table next to you in a restaurant in Washington or Boston, ask your fellow diners what they think. If stealthy Russian military forces were to take control of Estonia or Latvia, what should the United States do? Would they support sending Americans to fight for the survival of Estonia or Latvia?

Imagine, for example, an uprising of ethnic Russians in Estonia or Latvia, either spontaneously or at the instigation of Russian security services; a heavy-handed response by that nation’s weak police and military forces; an appeal by ethnic Russians to Putin to honor his “Putin Doctrine” declaration during the liberation of Crimea that he would come to the defense of ethnic Russians wherever they were attacked; an attempted replay of the hybrid war against Ukraine; and a confrontation with the battalion of six hundred American or NATO forces now on regular rotations through the Baltic states. Some Russians have gone so far as to suggest that this would provide sufficient provocation for Moscow to use a tactical nuclear weapon; Russia’s ambassador to Denmark, for example, recently threatened that Danish participation in NATO’s missile-defense system would make it “a target for Russian nuclear weapons.” What’s more, Russia is exploring stationing Iskander missiles in Kaliningrad—the Russian enclave between Lithuania and Poland—while Sweden’s intelligence has publicly stated that it views Russian intelligence operations as preparation for “military operations against Sweden.”

IN A climate of mutual suspicion further fueled by domestic politics on both sides, assurances of benign intentions rarely suffice. Christopher Clark’s 2013 book, The Sleepwalkers, provides a persuasive account of how, in the days preceding World War I, both alliances contemptuously dismissed the explanations and assurances they heard from the other side.

Of course, alliances are now Putin’s weakest point. Russia does not have a single ally committed to supporting Moscow in war. Nevertheless, one should be cautious about counting on Moscow’s isolation in a longer-term confrontation with the West. One reason Kaiser Wilhelm II presented his ultimatum to Russia was that he did not believe England would join Russia in a war over the crisis in the Balkans, where London had traditionally opposed Russian influence. Furthermore, without England, few expected France to offer much resistance. What those who count on Russian isolation today do not properly take into account is that a powerful and assertive alliance prepared to pursue its interests and promote its values inevitably stimulates antibodies. It was that sense of Germany’s determination to change the geopolitical balance in Europe and in the world that prompted Britain to depart from a century of splendid isolation and become so entangled with allies that when war came, it had little choice but to enter. It is the same sense that is leading China today to expand its ties with Russia during its conflict with the United States.

To be clear, there is virtually no chance that China would join Russia against the United States and Europe in a confrontation over Ukraine. Likewise, China is not prepared to bail Russia out financially or to risk its lucrative economic integration with the West to support Moscow’s revanchist ambitions. But neither is Beijing indifferent to the possibility of Russia’s political, economic or (particularly) military defeat by the Western alliance. Many in Beijing fear that if the United States and its allies were successful in defeating Russia, and particularly in changing the regime in Russia, China could well be the next target. The fact that the Chinese leadership views this as a serious threat could, over time, push Beijing closer to Moscow, a development that would fundamentally alter the global balance of power.

Moreover, if there were a Russian-American war, one needs to think carefully about what actions the Chinese might choose to take against Taiwan, for example, or even to punish neighbors like Japan or Vietnam whom Beijing believes are cooperating with Washington to contain its ambitions.

Neither China nor Russia is the first state to confront a powerful and growing alliance. Nor is the United States the first to receive enthusiastic appeals from prospective allies that can add marginally to overall capabilities, but simultaneously bring obligations and make others feel insecure. In a timeless passage in his History of the Peloponnesian War, Thucydides recounts the Athenian response to a troubled Sparta: “We did not gain this empire by force. . . . Our allies came to us of their own accord and begged us to lead them.” Needless to say, Sparta did not find that explanation reassuring—and that excuse did not prevent thirty years of war that ended with defeat for Athens, but at a price far beyond any benefits that accrued to the victor.

To recognize the potentially catastrophic consequences of war with Russia does not require paralysis in addressing the challenge of a resurgent but wounded Russia. The United States has a vital interest in maintaining its credibility as a superpower and in assuring the survival and security of its NATO alliance—and thus of every one of its NATO allies. Moreover, in international politics, appetites can grow quickly if fed by easy victories.

The Russian president’s currently limited objectives in Ukraine could become more expansive if Russia does not face serious resistance. After all, the smooth annexation of Crimea led to an outburst of triumphalist rhetoric in Moscow about creating a new entity, Novorossiya, which would include eastern and southern Ukraine all the way to the Romanian border. The combination of resistance by local populations, the Ukrainian government’s willingness to fight for its territory, and U.S. and EU sanctions quickly persuaded the Russian leadership to curtail this line of thinking. When a nation is prepared to fight for important interests, clarity about that determination is a virtue in discouraging potential aggression.

Yet the United States should be careful to avoid giving allies or friends—like Kiev—the sense that they have a blank check in confronting Moscow. During World War I, even such a strong supporter of the war as Pavel N. Milyukov—leader of Russia’s Constitutional Democrats and later foreign minister in the Provisional Government—was shocked at the lengths to which British foreign secretary Sir Edward Grey would go in refusing to assign any blame for the conflict to the Serbs. “Listen,” he reports saying to Grey, “the war started because of Serb grandstanding. Austria could think that it was in serious danger. Serbia was aspiring to do no less than to split Austria.” To Grey, however, an ally could do no wrong.

The Balkan crises in the several years prior to World War I deserve careful study. Few at the time could conceive that they would become the flashpoint of a fire that would eventually become a continental inferno.

But they did. Meeting the challenge of an angry but weakened Russia today requires a subtle combination of firmness and restraint. Where vital American interests are engaged, we have to be able and willing to fight: to kill and to die. Effective deterrence requires three C’s: clarity about red lines that cannot be crossed (for example, attacking a NATO ally); capability to respond in ways that will make the cost of aggression greatly exceed any benefits an aggressor could hope to achieve; and credibility about our determination to fulfill our commitment. At the same time, we should recognize that if American and Russian forces find themselves firing upon each other, this would violate one of the principal constraints both sides respected assiduously during four decades of the Cold War—risking escalation to a war both would lose.

Military force and economic warfare such as sanctions are indispensable instruments of foreign policy. When employed without a sound strategic vision and artful diplomacy, however, instruments of coercion can develop their own momentum and become ends in themselves. Having managed a confrontation over the Soviet Union’s attempt to install nuclear-tipped missiles in Cuba that he believed had a one-in-three chance of ending in nuclear war, President John F. Kennedy spent many hours reflecting on the lessons from that experience. The most important of these he offered to his successors in these words: “Above all, while defending our vital interests, nuclear powers must avert those confrontations which bring an adversary to a choice of either a humiliating retreat or a nuclear war.” It is a lesson statesmen should apply to meet the challenge Russia poses in Ukraine today.