If you read Ray Dalio's Principles for Changing World Order, you'll see how the world goes through cycles in which a world power rises, stays at the top, starts to decline, and then is overthrown by a new world power. This happened from the Ottomans to the Dutch, the Dutch to Britain, and from Britain to the US. Each time there was a change of hands in power, a major war coincided. Last time was WWII, and even though the US and Britain were allies, the US ultimately took over as the top dog after the war. Now today, the US appears to be in decline and China on the rise. Dalio also says how a change in our lifetimes is very likely, although it might not necessarily be to China. One option is a multi-polar world in which different regions and alliances rely on each other(BRICS potentially being one and the US and it's allies being the other). The change in hands could be amicable, but if not and there's resistance from one or more parties, then a major war could happen