( CNN) We need to talk. On that, both the United States and Russia can agree. And the opportunity of the G20 summit in Hamburg this weekend is ideal. It lets anything from a full, formal sit-down session to a staged informal encounter in which both sides invest minimal political capital. But don't get your hopes up.

In a rational world, the two leaders' agenda would be packed with must-solve problems. American and Russian forces, and their local proxies, are perilously close to conflicts in Syria. Frightening near-misses have become nearly routine over the Baltic Sea. A huge Russian military exercise, Zapad-1 7, looms in September, causing jitters in NATO's front-line nations of Estonia, Latvia, Lithuania and Poland.

The arms control regime bequeathed to the world at the end of the old Cold War is in tatters. The war in Ukraine grinds on, amid deadlocked diplomacy. Add sanctions, snoop wars and disagreements over the Arctic, and you have enough material for a series of summits , not just one meeting.

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