Russia has never stopped being an aggressor since the 1900s. They want to take over all of eastern Europe. They know that the United States will not offer any serious response to their aggression in the Ukraine. Once they have taken over the Ukraine they won't hesitate to march into the rest of Europe.
I think the US and Europe have made a grave mistake in underestimating Russia these last two decades. Russia is spreading its influence all over the globe and all Putin wants is to be the greatest superpower in the world. The missile hitting the MH17 is just one example of Russia flexing its muscles in covert ways.
Russia remains a serious threat to our security. Russian weapons have been smuggled into Ukraine to undermine the Ukrainian government. The Russian military is also propping up the Assad regime in Syria. Furthermore, the Russian military has been aggressively challenging U.S. forces, as well as sponsoring hacking attacks against American companies and interests.
Although Russia lost the cold war and is no longer a true super power, they are a huge threat to America's security simply due to the fact that they have nuclear weapons. If Russia launched a nuclear attack or provided enemies of the United States with nuclear weapons, it could result in a world war and massive devastation.