Russia’s annexation of Crimea has upended fundamental assumptions about European security in the post­–Cold War era. The use of force, violent nationalism, and land grabs are back in style, and a lengthy confrontation between East and West suddenly seems much closer at hand.

Apparently stunned by Russian belligerence, the United States and its European allies have been scrambling to find a way to deter further Russian aggression in Eastern Europe, including through warnings and limited economic sanctions. Few believe, however, that these will have any significant effect on Russian behavior. They will neither cause Putin and his government to withdraw from Crimea nor change Russia’s willingness to use military force to “protect fellow Russians abroad,” wherever they may be. In fact, the West’s befuddled response has only played into the Kremlin’s hands; the Kremlin looks powerful while Washington and Brussels appear impotent and divided.

With no good short-term options available for pushing the Russians out of Crimea or even preventing further incursions into Ukraine, the West would do well to consider a more robust long-term option to deter Russia from moving deeper into Europe. NATO should offer membership to Sweden and Finland, and Sweden and Finland should accept. These two countries are the most active members in NATO’s Partnership for Peace program. Since the initiation of the program in 1994, both Sweden and Finland have participated in its full range of activities, including joint exercises, disaster management training, and cooperation on science and environmental issues. In the operational field, both countries have contributed troops and resources to several NATO-led missions, including in Afghanistan, the Balkans, and Libya. Both countries, too, have long been considered prime candidates for NATO membership, but historical and domestic factors -- including long-standing policies of military nonalignment -- have prevented them from taking the plunge, and NATO has not insisted.

Expanding NATO to Sweden and Finland would achieve several important aims. From a political standpoint, it would bring the NATO border ever closer to Russia, demonstrating that military aggression in Europe carries major geopolitical consequences. Sweden and Finland’s nonalignment has offered Russia a comforting buffer zone along its northwestern border ever since the end of World War II. If Sweden and Finland were to join NATO now, that buffer would be gone, and the alliance would gain two of the world’s most democratic, politically stable, and economically successful countries. NATO would also pick up two very active proponents of transatlanticism that have consistently argued for strong U.S. involvement in Europe.

From a military standpoint, Sweden and Finland would add technologically sophisticated and well-equipped armed forces to the alliance. Over the past 15 years, the Swedish military has gained international respect for its ability to deploy well-trained and professional soldiers that can easily integrate with U.S., NATO, and European forces. Meanwhile, Finland's recent acquisition of airborne cruise missiles and modern battle tanks makes the Finnish armed forces among the best-equipped in northern Europe. At any rate, although relatively small in numbers, the Swedish and Finnish armed forces are already more NATO-compatible than the forces of many current NATO members. Even more important, Sweden and Finland’s formal inclusion in the alliance would finally allow NATO to treat the entire Arctic-Nordic-Baltic region as one integrated military-strategic area for defense planning and logistical purposes, which would make the alliance much more able to defend Estonia, Latvia, and Lithuania against Russia.

For Russia, the loss of Sweden and Finland to NATO would represent a serious geostrategic blow, one that would outweigh any gains made from the annexation of the Crimea. It would place NATO forces within arm’s length of Russia’s strategic nuclear submarine bases located on the Kola Peninsula. It would also turn the Baltic Sea into a NATO lake, one through which much of Russia’s vital trade and energy exports would have to transit. Indicating Moscow’s level of concern about NATO expanding to the North, Russian Prime Minister and former President Dimitri Medvedev stated in June 2013 that any expansion of NATO to Sweden and Finland would upset the balance of power in Europe and force a Russian response.

The historical significance of Swedish and Finnish membership in NATO would not be lost on the Russians, either. Sweden and Finland are more than just two small and peaceful neighbors along Russia’s northwestern periphery. Sweden is a historical enemy that fought Russia for control of the Baltic region for centuries before being dealt a decisive defeat by Russian forces in 1809. After that, Sweden formulated its policy of military nonalignment, now 200 years old, which kept it out of both World Wars and on the sidelines of the Cold War. However, in more recent times, Sweden’s very active promotion of independence for the three Baltic Republics in the 1990s and their later inclusion in NATO and the EU has not been forgotten in Russia, although Sweden itself has remained outside NATO.

Finland, in turn, is the country that got away, even though it is now a close trading partner of Russia. Taken from Sweden and annexed by Russia as an Imperial Grand Duchy in 1809, Finland declared independence in the turmoil of the 1917 Russian Revolution. Facing Soviet aggression in 1939, Finland was the only country in Eastern Europe to fight off the invading Soviet Red Army, reaching a very bloody stalemate with it in the Winter War of 1939–40. Finland subsequently joined Nazi Germany in an ill-fated attack on the Soviet Union in 1941. Ultimately defeated but never occupied by the Soviet Union, Finland acquiesced in 1948 to signing the Treaty of Friendship, Cooperation, and Mutual Assistance with its neighbor, an arrangement that became known as Finlandization. In 1992, after the collapse of the Soviet Union, Finnish foreign policy and Finland itself were at last truly independent. Finland joined the EU in 1995, but chose not to join NATO to avoid provoking Russia.

Although Moscow views both countries with a certain wariness born of a history of complicated geopolitical engagement, it is a history that has mostly favored Russia: both Sweden and Finland have long been careful not to openly challenge their bigger neighbor. From a Russian perspective, Swedish and Finnish membership in NATO would rewrite this past and would throw uncertainty into the future geopolitical relationship among the three countries.

Given the upsides, bringing Sweden and Finland into NATO seems like a no-brainer. But the two countries have to agree to it. To outside observers, their general unwillingness to join NATO might seem odd. During the Cold War, both pursued a combination of relatively strong territorial defense and foreign policy nonalignment aimed at reducing tensions between the superpowers. In Finland, this policy was rooted in the experience after World War II of successfully balancing living next to the Soviet Union while preserving a democracy and liberal market economy. In Sweden, the policy was more the result of its citizens’ idea that their country's golden post–World War II years of economic prosperity and international moral standing were closely tied to armed neutrality -- the notion that Sweden alone stands between the East and the West as an upholder of world peace. So far, none of the two countries’ big political parties have been willing to challenge these historical legacies. And, not surprisingly, both publics are staunchly opposed to NATO membership.

Yet in Sweden’s case, the official interpretation of the Cold War is not the whole story. Official armed neutrality was complemented by secret bilateral cooperation with the United States and select NATO countries that guaranteed Western support in case of a war with the Soviet Union. That duality worked for a long time, and Sweden’s political elites have been comfortable seeking security through informal bilateral ties to the United States and other European countries, rather than through official membership in NATO. Finland’s proximity to Russia and its strong tradition of self-sufficiency made its options more limited than Sweden’s.

Yet Russia’s 2008 invasion of Georgia rattled many countries, including Sweden and Finland. Still determined to stay outside NATO, the Swedish government unilaterally issued a solidarity declaration in 2009 stating that Sweden would support its Nordic and EU neighbors in case of disaster or armed attack. Sweden declared that it would expect similar support under similar circumstances. At the time, the declaration was received with skepticism, if not downright ridicule, and then largely ignored. But the Russian invasion of the Crimea has reignited Sweden’s concerns, which have not been helped by NATO Secretary-General Anders Fogh Rasmussen’s repeated warnings that although Sweden is NATO’s most active and most capable partner, it cannot count on assistance in the event of an attack -- after all, the alliance’s Article 5 security guarantee extends only to members of NATO. And the fact that Ukraine, similar to Sweden and Finland, is a long-standing member of NATO’s Partnership for Peace but did not receive any meaningful help in countering Russian aggression vividly underscores the upside of joining NATO. This message has not been lost on Finland either. The Finnish prime minister recently declared support for an open debate on NATO, indicating how seriously the Finnish political elite views the situation.

In other words, more than ever, both countries might be willing to join the alliance. Many in the political and military establishments in Sweden and Finland have grown increasingly positive about the idea. The sticking point is the general public, which remains more skittish. For this reason, it is unlikely that membership applications from Sweden and Finland will arrive tomorrow -- or even next week -- at NATO headquarters in Brussels. But, given the importance of all three joining forces, Western leaders should start booking flights to Stockholm and Helsinki to make the case that Sweden and Finland would not only be most welcome in NATO but that the countries have a responsibility to their own citizens -- as well as to the citizens of neighboring countries -- to become part of a long-term solution to counter Russia in Eastern Europe. The leaders of Sweden and Finland should listen carefully and should finally shed the legacies of the past in the pragmatic fashion that is the Nordic way.