Southern Spain tries to break away, Muslims from all over Europe and ME rush in and help, then take over and establish a Muslim state.
Britain gets involved to protect Gibraltar. In no time, it's a free-for-all.
European countries with significant numbers of refugees risk internal revolt.
The NATO pact fractures, and Russia sees an opportunity to retake the Baltic states. Poland is once again overrun and divvied up, by the same two countries as last time.
Switzerland attempts to remain neutral, not so successful this time. Sweden is an active participant, after losing control of its own government.
Greece falls quickly. Italy and France hold on a little longer. For a while, Britain stands alone, but Churchill is long dead and the Royal Family is impotent.
From here on it gets Biblical.
Camp of the Saints is a user manual!