I think this was originally a German/Polish joke but my dad always tells it like this (he figures his family was German-American, so we know he doesn't mean it).

WWI--there are two trenches on the battlefield, one full of Americans and one full of Germans. They've been throwing grenades at each other for hours and neither side is making much progress. Finally, one German asks another, "What's a common American name?"

"Bob," suggests the comrade, so the first German shouts:

"Hey, Bob!"

An American stands up in the opposing trench and says, "Yeah?" and the Germans shoot him. The Germans do this four or five more times until finally one of the Americans asks his neighbor, "Well, what's a common German name?"

"Fritz," says the second American. The first American hollers:

"Hey, Fritz!"

Thinking quickly, the German replies, "Is that you, Bob?"

"Yeah, it's me!" . . . [you get the picture]