Phone calls between relatively low-level diplomats are normally not newsworthy. But Monday's conversation between U.S. Undersecretary of State Victoria Nuland and Russian Deputy Foreign Minister Grigory Karasin on the simmering conflict in Ukraine is an exception. The bilateral nature of the conversation and its timing amid mounting claims of cease-fire violations from the Ukrainian government and separatist forces makes it uniquely significant. Moreover, it reaffirms that the evolution of the Ukrainian conflict — whether toward a settlement or toward escalation — will be most strongly shaped not by Kiev but by the actions of and relationship between Moscow and Washington.

Since the Ukrainian crisis started nearly 18 months ago, two negotiation formats in particular stand out among numerous talks and meetings. The first is the Minsk talks between representatives from the Ukrainian government, the pro-Russia separatists and the Organization for Security and Co-operation in Europe, which address the conflict on a tactical level. The other is the Normandy talks between representatives from Ukraine, Russia, Germany and France, which consider the conflict on a broader, political level. Notably absent from both talks, despite being a major political, economic and security player in Ukraine and the broader standoff between Russia and the West, is the United States. Washington has been diplomatically active in the conflict, but U.S. and Russian officials have met at various times only on an ad hoc basis.

However, this practice may have changed over the weekend, when Russian Presidential Chief of Staff Sergei Ivanov said in an interview that Russia and the United States had come to an agreement to set up a "special bilateral format" of talks between the two countries — talks that would involve Nuland and Karasin. In explaining the formal announcement, Ivanov said that expanding the Normandy format to include the United States would simply be too "risky," adding that the two countries would coordinate talks on Ukraine bilaterally "for the time being." Thus the phone call between Nuland and Karasin took place to discuss the implementation of the Minsk agreement and the constitutional reform process in Ukraine, with further discussions likely to follow.

The Ukraine conflict is at its core a conflict between two geopolitical imperatives. Russia wants to protect its interior by using its surrounding territories to establish a buffer. The United States wants to prevent the rise of regional powers that could potentially challenge U.S. hegemony. These imperatives collided in Ukraine, which of all the countries in the former Soviet periphery has the most strategic importance for modern Russia. If Ukraine supports Moscow, Russia becomes a regional power on the rise. If Ukraine supports the West, Russia becomes vulnerable from without and within. The Euromaidan movement of February 2014 reversed Russia's position from the former to the latter. Moscow responded by annexing Crimea and supporting the separatist insurgency in eastern Ukraine in a bid to undermine or neutralize Kiev's pro-Western government.

So far Russia's plan has been unsuccessful. Ukraine aligned itself even more closely with the West by pursuing greater economic and political integration with the European Union and greater security and military cooperation with NATO. Ukraine's close relationship with NATO is particularly worrisome for Russia, which has long feared the military alliance pushing up against its borders. Moscow has made multiple efforts to keep NATO's influence at bay, putting diplomatic pressure on Georgia in 2008 when Georgia declared its alliance with NATO, for example. It showed its concern about NATO even more dramatically in the conflict in eastern Ukraine. And of all the NATO countries, the United States has the strongest military and the most assertive policies challenging Russia throughout the former Soviet periphery.

Russia's long-held suspicion of U.S. influence in its periphery makes the decision to start regular bilateral talks a significant step. In some ways, these two countries wield more power to shape the political and military outcome in Ukraine than the Ukrainians and separatists themselves. But holding such talks does not necessarily indicate that a resolution or even a de-escalation of the conflict is imminent. Issues still divide the two sides, particularly what kind of autonomy Ukraine's central government should give the rebel regions.

All the major parties in the Ukrainian conflict support some level of decentralization, or the granting of greater powers to regional governments. The disagreement is over the timing and extent of the process. Russia sees decentralization as a way to maintain a buffer zone in the east outside of Ukraine's direct control, while Ukraine sees it as a way to compromise but still effectively retain control over the entire country. Ukraine wants to see separatists implement the Minsk agreement and lay down their arms before officials amend the national constitution to grant the eastern territories more regional autonomy. But separatists want the constitutional changes first, and they want a role in determining those changes. Only then, they say, can they fully implement the cease-fire.

Broadly speaking, the United States supports the Ukrainian position; Russia supports the separatists. However, during a recent visit to Ukraine and preceding her phone conversation with Karasin, Nuland weighed in on the Ukrainian legislature's debate over the constitutional amendment. Nuland urged Ukraine to give the country's eastern regions a controversial and highly debated "special status" under the law. Officials had not included the term in the constitutional amendment draft, but U.S. pressure to deliver more on the sensitive issue could be seen as a nod to Russia.

But Nuland's actions could also be a more nuanced effort to help Ukraine: The more substantial and unimpeachable Ukraine's constitutional reforms, the less room Moscow and the separatists have to criticize the changes and justify their own cease-fire violations. Washington has echoed Kiev in demanding that the separatists abide by the cease-fire, threatening Russia with more sanctions and — according to some leaked reports — restrictions on Moscow's access to credit, if separatists continue to violate the Minsk agreement.

Russia's reactions have also been mixed. The Kremlin has spoken somewhat positively of the reform process, but Russia is still influencing the Ukrainian battlefield while demanding more political concessions for the separatist territories. Russia is also seeking U.S. concessions on Ukraine for its help in facilitating the Iran nuclear agreement. Moscow and Washington are trying to reach an accommodation while keeping their threat options open as well. With more talks between Nuland and Karasin set to take place, the evolution of Ukraine's conflict and the political reform process will be the true test of the effectiveness of this new bilateral dialogue between the United States and Russia.