If you are playing a game and there are two MOBs, a boss MOB and a smaller mob, you take out the smaller mob first. Because the boss MOB takes a long time to kill, and during that long time you'd be taking damage from both of them. If you take out the smaller mob first, then you only take damage from both of them for a short while.

So you take out Russia first, then focus on China.

BTW: mob originally was short for "(self) mobile object" in MUDs.