Things just aren’t good on any front involving the relationship between the United States and Russia. The nearly-hot war between Russia and Ukraine, the annexation of Crimea and the downing of Malaysian Flight 17 are capturing the main headlines. But there’s other grim news as well.

First, the landmark arms control agreement signed by Reagan and Gorbachev seems to be falling apart thanks to Russian aggression.

The United States has concluded that Russia violated a landmark arms control treaty by testing a prohibited ground-launched cruise missile, according to senior American officials, a finding that was conveyed by President Obama to President Vladimir V. Putin of Russia in a letter on Monday.

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.

And now, even scientific and mutual defense cooperation between Russian and American nuclear scientists is decaying.

It was only 11 months ago that the American energy secretary — Ernest J. Moniz, a former M.I.T. professor who has championed scientific programs that would bury the Cold War competitions between the United States and Russia — went to Vienna to sign the agreement, an indication of how recently the Obama administration believed it had a chance of building on a quarter-century of gradual integration of Russia with the West.

Handshakes and congratulations exchanged with Mr. Moniz’s Russian counterpart, Sergey V. Kirienko, sealed an arrangement that would let Russian scientists into, among other places, the heart of the American nuclear complex at Los Alamos National Laboratory in New Mexico, where the first atomic bomb was constructed 70 years ago, and a dozen sister laboratories devoted to the making of the American nuclear arsenal. In return, American scientists would be allowed deep into Russian nuclear facilities, including the birthplace of the Soviet bomb.

The Energy Department’s announcement of the deal also highlighted its potential for “defense from asteroids,” shorthand for a proposal to recycle a city-busting warhead that could be aimed at an incoming earth-destroyer — a plot Hollywood had imagined 15 years before in two far-fetched thrillers, “Armageddon” and “Deep Impact,” in which Bruce Willis and Ben Affleck, among others, saved humanity.

Now the program has come to a complete halt as part of the post-Crimean sanctions regime placed on Russia by the United States. Both sets of scientists aren’t happy about the agreement falling apart — but as the New York Times notes: it is hard to exchange nuclear scientists when the rest of the US-Russian relationship has become so cold.