For resources and to prove that they were powerful. Their initial holdings before WW2 were not enough to fuel the war machine it wanted to become.
1. The Pax Mongolica (1 century of peace w/ neighboring people under mongol rule)
2. Unifaction of Russia
3. The Pax Mongolica reopened of silk road routes from Asia to Europe
6 agreement by which a knight agrees to be a sub-vassal to vassal in exchange for land