Well, if WWIII turns out to be an alien invasion, then Earth would end up as a colony of some alien race. However, humanity will hide out in the mountians and jungles, engage in guerilla warfare, adapt alien technology to its own, and eventually drive away the aliens. That is unless humanity has developed colonies of its own throughout the solar system by the time aliens arrive as well as a fleet of nuclear powered interplanetary spaceships.
But I think WWIII will occur after some new power blocs are formed.
China will create a "Bejing Alliance" with Cuba, North Korea, Venezuela (ruled by the leftist Hugo Chavez), along with a few leftist dictators in Africa as well as support Marxist groups like FARC in Columbia and the Shining Path in Peru.
Unless Iran has experienced another revolution from its young people, the ruling regime will try to unify with nations like Georgia, Armenia, Azerbajan as well any nation ending in "stan" creating a Restored Caliphate of Islam. The RCI will then form alliances with Sudan, Indonesia, and Malaysia.
Egypt, Lybia, Syria, Yemen could unify to restore the United Arab Republic.
Russia and India will maintain their alliance against China and Pakistan.
Anyway, WWIII could start like this.
First, North Korea invades South Korea. The USA, EU, Japan all send reinforcements to beat back the North Koreans. Meanwhile, China launches an invasion of Taiwan but is repelled by the combined fleets of the USA and Taiwan. China sends one million troops into Korea and the Korean War escalates.
Meanwhile, hostilites erupt between India and Pakistan and China comes to Pakistan's aid and two million Chinese and Indian troops are fighting each other.
The Restored Caliphate of Islam moves into Afghanistan as well as against the Gulf States. The United Arab Republic also moves against the Gulf States. Faced with a two-front war, the USA and EU commit troops to the Gulf States defense.
Meanwhile, SRBMs are launched from hidden missile silos in Venezuela against the USA destroying Miami, New Orleans, Houston, Corpus Christi, and Orlando. FARC, armed with Chinese weapons, takes over Columbia while the Shining Path takes over Peru.
With the Russians supporting Pakistan, China unleashes ten million troops armed with AK-47s into Siberia. Meanwhile, The RCI also sends one million troops into Central Russia.
With WWIII being declared, America is put on a wartime production level, the draft is reinstated, and America begins to build a military of 25 million strong.