The worst case scenario is this:
The alliance system basically caused WWI because many countries were obligated to respond in defense of allied countries whether they wanted to or not. In the end, one assassination cost millions of lives due to strategic alliances.
China and Russia have a lot of alliances with countries in opposition to us, especially in the Middle East. In my opinion, the defense alliances between China, Iran, Venezuela, Cuba, and North Korea will be the igniting spark of WWIV.
But what about WWIII?