Takes Place Somewhere in Europe?

According to the Half-Life wikia, HL2 and its episodes take place "somewhere in Europe" (City 17, etc.). But how come all the characters from HL1 have suddenly moved from the US to Europe in the 20 year span between the games? Is it ever really explained? Did the Combine move all the population of America to Europe?

-Godard: What brought you to Hollywood?
-Ford: A train.