So who do you see as the aggressor here?
That's actually complicated. The original aggressor was the Western-backed Maidan revolution which overthrew Ukraine's elected government. One of the consequences of that coup was that Russia's ally was replaced with a hostile government in Kiev, prompting Russia to occupy Crimea in order to secure the strategic Russian naval base at Sevastopol. The Russian occupation was an offensive action, but a defensive move.
I am not arguing who is right or wrong in this conflict, just weighing the strategic interests of the parties involved. To Russia, securing Crimea (and possibly even Ukraine, if Poland becomes a NATO member) is a vital necessity and worth going to war over. Neither Crimea nor Ukraine is vital to the defense of America or Western Europe, and is therefore not worth going to war over.