The Yugoslav War and NATO's move east convinced Moscow that the Alliance was surrounding Russia with potential adversaries, and the deployment of anti-missile systems (ABM) -- supposedly aimed at Iran's non-existent nuclear weapons -- was seen as a threat to the Russian's nuclear missile force.
One immediate effect of ABMs was to chill the possibility of further cuts in the number of nuclear weapons. When Obama proposed another round of warhead reductions, the Russians turned it down cold, citing the anti-missile systems as the reason. "How can we take seriously this idea about cuts in strategic nuclear potential while the United States is developing its capabilities to intercept Russian missiles?" asked Deputy Prime Minister Dmitry Rogozin.
When the U.S. helped engineer the 2014 coup against the pro-Russian government in Ukraine, it ignited the current crisis that has led to several dangerous incidents between Russian and NATO forces -- at last count, according to the European Leadership Network, more than 60. Several large war games were also held on Moscow's borders. Former Soviet president Mikhail Gorbachev went so far as to accuse NATO of "preparations for switching from a cold war to a hot war."
In response, the Russians have also held war games involving up to 80,000 troops.
It is unlikely that NATO intends to attack Russia, but the power differential between the U.S. and Russia is so great -- a "colossal asymmetry," Dmitri Trenin, head of the Carnegie Moscow Center, told the Financial Times -- that the Russians have abandoned their "no first use" of nuclear weapons pledge.
It the lack of clear lines that make the current situation so fraught with danger. While the Russians have said they would consider using small, tactical nukes if "the very existence of the state" was threatened by an attack, NATO is being deliberately opaque about its possible tripwires. According to NATO Review, nuclear "exercises should involve not only nuclear weapons states...but other non-nuclear allies," and "to put the burden of the doubt on potential adversaries, exercises should not point at any specific nuclear thresholds."
In short, keep the Russians guessing. The immediate problem with such a strategy is: what if Moscow guesses wrong?
That won't be hard to do. The U.S. is developing a long-range cruise missile -- as are the Russians -- that can be armed with conventional or nuclear warheads. But how will an adversary know which is which? And given the old rule in nuclear warfare -- use 'em, or lose 'em -- uncertainty is the last thing one wants to engender in a nuclear-armed foe.