In late 2009, the Holy See and the Russian Federation agreed to full diplomatic relations at the ambassadorial level, bringing the total of such exchanges to 178 -- a remarkable achievement, considering that, in 1978, the Holy See had full diplomatic relations with only 84 states. Less than a hundred years after the Entente powers banned the Holy See from the post-World War I peace conference by a secret clause in the Treaty of London that brought Italy into the war on the side of Great Britain and France, the Holy See -- the juridical embodiment of the ministry of the Bishop of Rome as universal pastor of the Catholic Church -- is fully engaged in the complex worlds-within-worlds of international diplomacy.

Those complexities just became, well, more complex, thanks to some distinctive features of contemporary Russian history and one long-standing feature of Russian culture. The latter is the close relationship between the Russian Orthodox patriarchate of Moscow and the Kremlin, which has endured through czars, commissars, and now presidents and prime ministers; the former involves the strange post-Cold War situation of Russia.

To say that Russia has never come to grips with the legacy of seventy-four years of communism is to understate the problem. Lenin's mummy -- the ghastly relic of one of the 20th century's greatest mass murderers -- remains on display for the veneration of the obtuse and the confused in Red Square. Parades celebrating the birthday of Stalin, whose homicidal record topped Lenin's, are not uncommon. Documentary film-makers who dare to tell the truth about communism's depredations are burned in effigy. History is re-written in order to mask, even deny, the horrors of the GULAG system (which, as Anne Applebaum demonstrated in her Pulitzer Prize-winning book, was not an accidental feature of Stalinism but an essential component of Stalinist "economics").

Vladimir Putin, the true center of power in Russia despite having been compelled to trade the presidency for the office of prime minister, has made it clear that he is not satisfied with a Russia shrunk to the country's size at the time of Peter the Great. Yet neither Putin nor his successor as president, Dmitry Medvedev, seems much interested in dealing with Russia's colossal demographic and public health problems, which include a rapidly shriveling native population (thanks to catastrophically low birth rates and declining life expectancy, both exacerbated by environmental degradation and rampant alcoholism). Meanwhile, Russia's "market" economy resembles a Mafia operation rather more than the "free economy" of which John Paul II wrote in Centesimus Annus.

The flashpoints in Putin's efforts to reconstitute the old Soviet "near abroad" as de facto or de iure parts of a Greater Russia are clear: the Caucasus, central Asia, and Ukraine. Ukraine is the strategic key to all the rest; without Ukraine, Russia cannot be a superpower. One of the chief repositories of Ukrainian national consciousness is the Ukrainian Greek Catholic Church, Byzantine in its liturgical life and polity but in full communion with Rome. Declared illegal under communism (in a brutal 1946 maneuver aided and abetted by the Russian Orthodox Church), the Greek Catholic Church of Ukraine was the world's largest underground religious community for four and a half decades. Its flourishing after communism, and its dedication to building a Ukraine that models the free and virtuous society proposed by Catholic social doctrine, is one of the most heartening stories unfolding in the former Soviet Union.

Vatican diplomats and ecumenists have had their difficulties with the Ukrainian Greek Catholic cause -- in part because of Ukrainian passions and indiscretions, but also because of a tendency to bend over backwards towards the Russian Orthodox patriarchate of Moscow for ecumenical reasons. But now comes the diplomatic rub. There is little reason to think that the patriarchate of Moscow will be anything but a willing, indeed enthusiastic, partner in any effort by the Russian state to reconstitute Greater Russia. If, at some point, Putin & Co. try to ingest a large chunk of eastern Ukraine, the Holy See's diplomats are going to face an enormous challenge, with grave implications for internal Catholic unity, ecumenism, and international relations.

George Weigel is Distinguished Senior Fellow and William E. Simon Chair in Catholic Studies at the Ethics and Public Policy Center in Washington, D.C.