It is the most serious allegation of an arms control treaty violation that the Obama administration has leveled against Russia and adds another dispute to a relationship already burdened by tensions over the Kremlin's support for separatists in Ukraine and its decision to grant asylum to Edward J. Snowden, the former National Security Agency contractor.
At the heart of the issue is the 1987 treaty that bans American and Russian ground-launched ballistic or cruise missiles capable of flying 300 to 3,400 miles. That accord, which was signed by President Ronald Reagan and Mikhail S. Gorbachev, the Soviet leader, helped seal the end of the Cold War and has been regarded as a cornerstone of American-Russian arms control efforts.
In his letter to Mr. Putin, delivered by the American Embassy, Mr. Obama underscored his interest in a high-level dialogue with Moscow with the aim of preserving the 1987 treaty and discussing steps the Kremlin might take to come back into compliance. Secretary of State John Kerry delivered a similar message in a Sunday phone call to Sergey V. Lavrov, the Russian foreign minister.
NATO's top commander, Gen. Philip M. Breedlove, has said that the violation requires a response if it cannot be resolved. "A weapon capability that violates the I.N.F., that is introduced into the greater European land mass, is absolutely a tool that will have to be dealt with," he said in an interview in April. "It can't go unanswered."
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