First it starts in the US. Total economical collapse due to a slowdown in the cash flow thru the economy.

The US might decide to get rid off some of it military to get its economy going again. Leftist might buy them and start a revolution. It will not go well and the US will descend into chaos.

Next the middle east lights up. With the US gone civil war arises almost immediately. This sends ripples all over the globe. Right-aligned groups in Germany, who where already on the edge, loose it and sail into refugee camps with machine guns.

How far they'll go after that is anyone's guess, they might decide to overthrow the government as well and bring back fascism.

The new nation that forms from the war in the middle east will be upset and in blind rage may declare war on Germany.

France and the rest of the western nations will either join Germany, or declare war against it depending if the middle east or Germany are on the moral high ground.

If France joins Germany, Russia will declare war on Germany, if not they'll join Germany. The rest of the world follows, picking a side, as the world descends into world war 3...

China has a chance of being natural or going after the ruins of the US.