In summary, U.S. presidents, Democratic and Republican, broke a 1990 promise to Russia not to extend NATO eastward to the Russian border, a promise made in return for Russia's pledge not to interfere with the reunification of Germany and its membership in NATO. (We may well ask why NATO continued after the Warsaw Pact and then the Soviet Union disappeared.) The US promise was broken when the members of the defunct Warsaw Pact and the Baltic states were incorporated into NATO, and two former Soviet Republics, Georgia and Ukraine, were widely discussed as future NATO members. (This has not happened yet, but the US government has intervened in both countries to keep them from being too close to Russia. In Ukraine this took the form of a coup, ousting an elected president, however objectionable he was.)
Since NATO was formed in opposition to the Soviet Union and its alliance, the expansion to the Russian border was reasonably interpreted as threatening. That a strong man--Putin--emerged to push back against American military provocation and economic intervention in the Russian economy should have been expected.
NATO’S widening umbrella doesn’t justify Putin’s bellicosity or his incursions in Ukraine or Georgia. Still, the evidence suggests that Russia’s protests have merit and that U.S. policy has contributed to current tensions in Europe.Western provocation of Russia is dangerous. NATO holds military exercises close to the Russian border, and it wouldn't take much for an "incident" to lead to a crisis and worse. Instead of engaging in moves and counter-moves, the two powers should be talking about dismantling their nuclear arsenals.