In WWI, when the USA joined the war, Germany wanted a quick victory and treaties made before the USA became fully functional (its troops were subpar by world standards). In order to do so, Germany planned to take out Russia first so it could then move all its armies to defeat France and then move to attack England. In order to take the Russians out, the Germans had a plan. Vladimir Lenin had been sitting in exile from Russia in Switzerland for quite some time. He was an influential Russian who wanted to create a socialist revolution. So what the Germans did is they brought Lenin back to Russia and funded the Bolshevik party. A civil war in Russia ensued (causing Russia to sign a treaty) and Germany successfully moved its troops onto the West. Unfortunately the plan stopped working after this point, mainly due to the blockade of trade by the allies and lack of steel. So basically, Germany is the reason why the USSR was formed.
I love the story of the infamous "sealed train"... I would say the ineptness of the Mensheviks and the noob abdication by Nicholas II were also contributing factors to Bolshevik rise.