Judging solely on the basis of personality, Pope Francis and Russian President Vladimir Putin may seem an odd geopolitical couple. Francis is a man of compassion and peace, while Putin is quite possibly the single world leader you most wouldn’t want to run into in a dark alley.

Yet when Francis and Putin meet on Wednesday in the Vatican, it will bring together two figures who’ve forged an improbably strong partnership. What remains to be seen is whether the potential fly in the ointment between Rome and Moscow ruins the party: Ukraine.

First, however, consider the areas where Francis and Putin have managed to do business.

Syria

In September 2013, the drums of war were beating as the United States and other major Western powers geared up to deploy military force against the regime of Bashar al-Assad in Syria, based on claims that Assad had used sarin gas against his own people.

Given Russia’s role as a patron of Syria, Putin was strongly opposed. Yet his position arguably was less consequential than that of Pope Francis, who convened a special day of prayer and fasting for peace in Syria and denounced an expansion of the conflict on multiple occasions.

By all accounts the pope’s line made a difference, for which Putin was grateful.

Former Australian Foreign Minister Bob Carr recently published a set of memoirs revealing details of a September 2013 meeting of the G20 in St. Petersburg, where Syria was the main topic of conversation. Francis had written to Putin as the summit’s host beforehand, urging member nations to “to lay aside the futile pursuit of a military solution.”

Carr reports that Italy’s prime minister at the time, Enrico Letta, said that while he’d like to support a strike, the pope’s stance “is a big factor for me domestically.” Letta insisted on having United Nations authorization, and with Russia and China as permanent members of the Security Council, Letta knew that by making UN approval a deal-breaker, he was effectively saying no.

Later, Carr recounted Putin’s remarks to the other heads of state. As he tells it, Putin ended by saying “We might listen to the pope,” and then quoted from Francis’ letter.

“Bang!” Carr wrote. “Cop that!” His point was that by being able to cite Pope Francis, Putin for all intents and purposes closed the argument.

Christians in the Middle East

Given that the majority of the embattled Christians in the Middle East are Orthodox, Putin insists that Russia has a special historical role as their protector. That may seem an odd stance for a former agent of the officially atheistic Soviet state, but it’s actually a return to Russia’s 19th-century role as the principal benefactor of Orthodox Christians in the Middle East, just as France and Austria backed the region’s Catholics.

“As regards the Middle East and its Christians, their situation is dire,” Putin said in April. “The international community is not doing enough … this is the motherland of Christians. Christians have lived there from time immemorial, for thousands of years.”

In some corners of the Middle East, such as the Syrian region of Qualamun, Russia actually has floated the idea of granting citizenship to pockets of Orthodox Christians, effectively offering them a security blanket.

Middle Eastern Christians “have known for centuries that no other country would look after their interests in the same way Russia would,” said Archpriest Nikolaj Balashov, the No. 2 man of the Patriarchate’s Department for External Church Relations, in 2013.

Aside from any spiritual motive, Putin also gains a clear political benefit from posturing as a champion of Christians in places such as Syria and Iraq, because it gives him cover for flexing Russia’s muscle.

Since Francis’ election in March 2013, meanwhile, no social or political issue has engaged the pontiff like the plight of persecuted Christians, especially in the Middle East.

In March, he demanded that the world stop trying to “hide” the reality of anti-Christian violence, and he’s also argued that the shared suffering of Orthodox, Catholics, and Protestants alike is the basis for a contemporary “ecumenism of blood.”

The possibilities for Russian/Vatican partnership were reflected in a first-ever resolution on protection of Christians in the Middle East adopted by the UN in Geneva in March, an initiative co-sponsored by the Vatican, Russia, and Lebanon.

In the run-up to Wednesday’s meeting, Russia’s Interfax news agency reported that government officials see the protection of Middle East Christians as a key agenda item for both Putin and the pope.

Cuba

Although Cuba no longer has the strategic significance for Russia it held during the Cold War, it remains among Moscow’s best friends in the Western hemisphere.

Russia recently forgave a reported 90 percent of Cuba’s debt, worth an estimated $32 billion, and signed a series of energy and trade deals. Russia also announced plans to reopen a military base in Cuba which, theoretically, could expand its ability to intercept American radio and telephone communications.

As a result, Russia feels a vested interest in seeing Cuba re-enter the community of nations and expand its regional influence.

Pope Francis, of course, played a lead role in paving the way for the restoration of diplomatic relations between Cuba and the United States, and has announced that he’ll add a stop in Cuba on his way to the US in September.

Francis and Putin thus have a shared interest in Cuba’s development, although depending on how things play out, they could end up as rivals for hearts and minds. In early May, after all, Raul Castro said that if Francis keeps going the way he is, even he might return to the Catholic Church.

Catholic/Orthodox relations

There’s long been a tight relationship between the Kremlin and the Orthodox Patriarchate of Moscow, with the Orthodox lending the government moral backing and the government in turn subsidizing the church and upholding its traditional privileges.

As a result, Putin sees promoting the Russian Orthodox Church as a policy priority, supporting anything that seems to enhance its prestige and international influence. He’s called for the church to play a larger role in citizens’ social lives, funded enhanced religion classes in state schools, and backed television programs emphasizing religious values.

Putin has found a friend in that regard in Pope Francis, who has made the push for Christian unity a cornerstone of his agenda, beginning with outreach to Orthodox Christianity.

Granted, Francis’ main interlocutor in the Orthodox world tends to be the Ecumenical Patriarch of Constantinople, Bartholomew I, who’s sometimes seen as a rival to Moscow in terms of leadership.

Yet the pontiff is also a realist, and he knows full well that of the roughly 225 million Orthodox Christians in the world, two-thirds of them belong to the Russian Orthodox Church. If he’s serious about unity, therefore, all roads run through Moscow.

Among other gestures of friendship, Francis invited a representative of the Russian Orthodox to speak at last October’s Synod of Bishops – who used the occasion, naturally, to complain about the Greek Catholics in Ukraine.

The trouble in Ukraine

If there’s anything with the potential to upend the spirit of common cause between Putin and Francis, it’s clearly Ukraine, where the critically important Greek Catholic minority believes the pontiff risks sacrificing the country’s welfare to pursue a policy of détente with Russia.

Many Ukrainian Catholics felt the pontiff went too far in that direction in early February, describing the violence in eastern Ukraine as “fratricidal,” when many Ukrainians would say it’s the product of Russian aggression.

“It shows the ignorance of the pope about the situation in Ukraine,” said Anatolij Babynskyj, editor of a prominent Greek Catholic journal, who blamed “pro-Russian forces at the Vatican” for distorting the pope’s view.

Right now, eastern Ukraine is an active combat zone. A senior US army officer recently estimated there are 12,000 Russian soldiers, including advisors, weapons operators, and combat troops in the area, all supporting Russian-instigated separatist forces.

“I think of you, Ukrainian brothers and sisters, this is a war between Christians,” Francis said in February. Calling the conflict in Eastern Ukraine a scandal, he pointed out that all those involved have the same baptism.

Francis urged prayer, saying “prayer is our protest before God in times of war.”

To Ukrainian Catholic ears, to be frank, those lines sounded uncomfortably like appeasement. The clincher was that Patriarch Kirill of Moscow and All Russia, head of the Russian Orthodox Church, publicly extolled the Vatican’s “balanced approach” while in the same breath accusing Greek Catholics of a “nationalistic” and “Russophobic” line.

Archbishop Sviatoslav Shevchuk, head of the Greek Catholic Church, said at the time that pope’s words were “particularly painful for all the people in Ukraine” and that they “reminded us of Soviet propaganda.”

Shevchuk is in a good position to get Francis’ attention, since he served as head of the Greek Catholic diaspora community in Argentina from 2009 to 2011 and developed a friendship with the future pope.

It will be fascinating to take the temperature of both the Vatican and the Kremlin after Wednesday’s tête-à-tête, to gauge whether the era of good feelings between Putin and Francis is still rolling — or whether Shevchuk and his church succeed in persuading the pontiff to strike a more hawkish stance.