Putin has threatened that Poland and Romania are now “in the crosshairs” of Moscow, over Poland and Romania's decision to build a NATO missile shield bases in their territory.
That missile defense shield comprises of the Standard Missile 3, or SM-3 as it’s called, and is purely defensive. It works not by blowing up a missile in midair but by slamming into it with great force; in other words, it couldn’t be turned into an offensive weapon, even if some future Western leader wanted it to be.
But that doesn't stop Putin and Russia from calling this a "threat and provocation" by the west against Russia, which has threatened over the years to launch missiles at Europe and even also went as far as to the have Russian army simulate a nuclear strike on Warsaw and on Poland as part of their military exercises.
President Vladimir Putin has for many years described NATO's efforts to develop defenses against ballistic missiles as a move to "neutralize" Russia's strategic nuclear deterrent and gain a "decisive military superiority." He said in November that Russia would respond by developing "strike systems capable of penetrating any missile defenses."
For Putin, not being able to threaten somebody, is considered a threat to Russia.