Missile Defense
Russia’s conventional military power has wildly atrophied since the Cold War ended, so the focus is solely on its strategic deterrent to meet all the required security needs. That’s mainly why Russia decided to withdraw its “no first use” pledge, and now tries to use nuclear weapons to “de-escalate” conflicts.
However, the wide majority of Russia’s nukes are based on ballistic missiles, especially land-based ballistic missiles. That’s exactly why missile defense is such a harrowing prospect for Russia. Of course, Russia was particularly irked by the prospect of the U.S. sending a missile-defense system to Europe, especially to counter Iran’s ballistic-missile arsenal.