Today's borders can be strong or weak, visible or hidden. In territories as diverse as the United States, Israel and Thailand, walls are built to keep out unwanted immigrants or to discourage terrorist incursions. Elsewhere, movement is regulated by passport and visa controls. The European Union allows free internal movement to its citizens while denying or controlling access for those from beyond its borders. Yet the realities of the modern world challenge the notion of national frontiers.

A recent journey around the Baltic States and parts of Russia brought this into focus. A traveler passes freely between the frontiers of Estonia, Lithuania and Latvia: These former Soviet republics are now fellow members of the EU and signatories to the Schengen agreement allowing unrestricted cross-border travel. Entering Russia, however, one faces strict controls. Visas are required and passports are scrutinized at length. Even in post-communist days, one has the sensation of going behind the Iron Curtain. Yet in another sense, that boundary is more permeable and more ambiguous.

The modern borders of Estonia and Latvia are roughly equivalent to those they had when they last were independent states, before World War II. But those boundaries no longer coincide with demographic realities. During the Soviet era, Russians were encouraged to emigrate to the Baltic region to dilute its ethnic homogeneity. Today, Narva, the easternmost city in Estonia, is 90 percent Russian-speaking; so too is Daugavpils, the second-largest city in Latvia. Many Russians in the Baltic States are non-citizens, denied the right to vote or work for the civil service.

Nevertheless, at least these Russian citizens can reside in the Baltic States. In the past, others were not so lucky. In Russia, I visited the cities of Vyborg and Kaliningrad. Like the Baltic States, these were incorporated into the Soviet Union at the end of World War II. Before the war, Vyborg was Viipuri, the second-largest city in Finland; Kaliningrad was Konigsberg, the German city once home to philosopher Immanuel Kant. Stalin's forces conquered the former in 1944; the latter was ceded to the Soviet Union according to the terms of the Potsdam Agreement. The inhabitants of the two cities were expelled by the victorious Soviets and today the population of both is almost wholly Russian.

Readers in Japan may know that there is another disputed territory beyond the other end of the vast Russian landmass. The island of Sakhalin, north of Hokkaido, was once divided between Russia and Japan. Stalin declared war on Japan in August 1945 in order to seize it completely, and to conquer the Kuril Islands (Chishima) off eastern Hokkaido. Again, the prewar population was expelled. The Japanese government has never relinquished its claim to the southern Kurils. More than six decades later, Russian control of these territories prevents the countries from signing a treaty to end the war.

Despite such continuing disputes, those borders now seem stable. There is little prospect of Japan regaining the Kurils any more than of Finland taking control of Vyborg or Germany of Kaliningrad. But other national boundaries are in flux. My trip, planned long in advance, happened to coincide with Russia's invasion of Georgia. In the week I arrived in Kaliningrad, the Russian Parliament formally recognized the two de facto independent breakaway regions of Georgia, Abkhazia and South Ossetia. With Russian troops still in undisputed Georgian territory as I write, it is unclear what this means for the integrity and sovereignty of Russia's southern neighbor.

Such uncertainties are mirrored elsewhere in today's world. Earlier this year, the former Yugoslav province of Kosovo, historically part of Serbia but today populated mainly by ethnic Albanians, declared independence, and has been recognized to date by nearly 50 nations. Sadly, even the EU failed to agree on a common response to Kosovo's secession; the new state is not recognized by EU members such as Spain, itself fearful of separatist sentiments in regions such as Catalonia and the Basque country. The question of how to deal with competing territorial claims remains a vexed one, especially when the widespread movement of people means that geographical frontiers no longer coincide with national or religious identities.

Did my travels suggest any answers? Perhaps Vyborg and Kaliningrad did so. The streets of Vyborg were busy with Finnish tourists, visiting a part of their historic homeland. Kaliningrad, likewise, is a tourist destination for Germans. No doubt many of these visitors are nostalgic or bitter. But it is possible for them to visit because the postwar German and Finnish governments took the pragmatic decision to relocate displaced citizens to territory under their control. Japan did likewise with refugees from Sakhalin and the Kurils. In consequence, human suffering was minimized and today these disputes are resolved at least to the extent that the countries involved can sustain some form of civil relations. Perhaps such compromise is necessary in a less than ideal world.