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And three recent examples of such political use of visas - one by Estonia, one by the Russian Federation, and one by the United States - highlight just how political the issuance of this type of travel documentation has become.
On Sept. 11, Estonia unilaterally introduced a full-visa regime at the Russian-Estonian border, ending the previous simplified regime for those living near the border.
Russian officials were angered. Aleksandr Safronov, the Russian Federation's consul general in Narva, complained that this action had a negative impact on divided families whose members will now have to apply for regular visas.
But Estonian officials justified this action as a necessary step to bring Tallinn into compliance with the requirements of the agreement that allows the free movement of people within the European Union, a body Estonia hopes to join.
Estonia's decision to require visas from Russians living just over the border in order to demonstrate its readiness to join a visa-free regime elsewhere is an example of a trend that is becoming the hallmark of post-Cold War Europe - the erection of a higher wall in one direction in order to dismantle one in another.
Also this week, Russian officials continued to offer explanations of the impact of Moscow's decision to withdraw from the visa-free regime among members of the Commonwealth of Independent States.
A Foreign Ministry spokesman reiterated that this decision, taken at the end of last month, was not intended to set up new barriers and that in any case the Russian government would deal with the question of visa requirements on a bilateral basis with its CIS partners.
The Russian government is completely within its rights to take this decision just as Estonia is. But observers in several of the countries most likely to be affected naturally viewed Moscow's decision less in terms of its effect on Russia than in terms of its impact on them.
By introducing a visa regime, the Russian government will now be able to deport or threaten to deport the numerous non-Russians who have moved to major Russian cities since the collapse of the Soviet Union.
That will certainly please many Russian nationalists who are upset about the presence of so many non-Russians in their midst. But more important, it will give Moscow powerful leverage over some of the CIS states.
The Armenian and Georgian economies, for example, depend heavily on transfer payments sent home from Russia by Armenian and Georgian workers now in the Russian Federation.
By ending or threatening to end these visa-free arrangements that allowed this situation to develop, Moscow can put enormous pressure on either or both governments to adopt a more cooperative posture with Russia on a variety of issues.
If, as now seems likely, Moscow continues the visa-free regime with Armenia but ends it with Georgia, Armenia will benefit and Georgia will suffer - unless, of course, Tbilisi is able to come to some arrangement that will allow for the restoration of a visa-free state.
And that possibility seems to many in the Caucasus and elsewhere to be one of the most important facts behind Russia's decision to withdraw from what had been one of the few generally acknowledged achievements of CIS.
Yet another example of the political use of visas came at the end of August when Washington refused to grant entry visas to Cuban and Yugoslav parliamentarians to attend a U.N.-affiliated conference in New York. The United States justified its decision not to admit them by pointing to the actions of both these individuals and their respective governments.
This decision was generally popular in the relevant ethnic communities inside the United States, and some commentators in Europe were quick to suggest that the visa denial in this case was first and foremost an election year gimmick.
But it also drew fire from the Russian government, which argued in a Foreign Ministry statement that the U.S. decision was "a flagrant violation" of American obligations as a host government of the United Nations Organization.
The Russian government added that "Moscow hopes the United States will fulfill its international obligations and create normal conditions for representatives of all nations to work at the U.N."
In each of these three cases, the governments involved acted within their rights as sovereign states, but they also did more than simply control who could enter their national territories.
They delivered a clear political message of their own intentions, and they helped promote their own political goals. Because other countries are likely to learn from these examples, more of them are likely to try to use visas in the same way.
And to the extent they do, something that had been a relatively simple travel document in the past is likely to be transformed into a political weapon with untold consequences for international cooperation.