It's been a day since I posted this, so I guess a bump is in order.
A link to a picture of the flag might have been a bit more inspirational!
I guess it would look something like the flag of Norway but with black instead of blue?
But that would look too much like the Norweigian flag at a distance--and closer up, too much like various Nazi regime era banners!
A picture might really clear things up, y'know?
Not sure what POD you'd need to get one of the nordic-inspired flags adopted, but for some reason I can't picture East Germany using the Weimar Flag, due to its non-communist origins. Maybe something incorporating the old Communist Party of Germany star instead.
http://en.wikipedia.org/wiki/File:KPD-logo.svg
But the DDR did use the Weimar flag, OTL, but with an extra logo superimposed--hammer-calipers thing with wheat cobs surrounding it, to indicate unity of workers on the land, by hand and by brain or some such.
Remember, the plan was for Germany to eventually come out of occupation as a united (if truncated) nation and the Soviets were hoping to win over the whole country--if not as a satellite, then as a neutral buffer state. The notion that the Western and Soviet occupation zones would wind up being separate nations was not in the air in 1945.
Also, the Weimar flag was not a creation of the Social Democratic party, it was inherited from the older revolutionary tradtions of Germany dating back to 1848; all Marxists, and many decidedly non-Marxist liberals, had a claim on it. Backing the Weimar flag was a way for the Soviets to try to appeal to a broad range of Germans without in any way conceding to the SDs. Whereas using the banner of the KPD would have been a, um, red flag in their faces.
Had Stalin had the foreknowledge that the Eastern zone he controlled directly would be all the German territory the Eastern bloc would ever get and every other bit of German soil would be against the Warsaw Pact anyway, maybe he might have gone with the red banner. But that would also be an early challenge to the Western powers to come and do something about it.
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I think I've answered adequately why the East German flag was in fact a variant on the Weimar tricolor; for the West to adopt a different flag, first of all I'd like to see the thing because as I said my best guess as to what it might look like is too suggestive of a continuation of the Prussian mentality and too similar to the Nazi look; maybe it would look very different--show me!
But a Nordic-cross design, even if it didn't look too much like the flags the Allies had fought in two wars already, wouldn't be a good look for German unity, north-south. Too redolent of Northern domination.
Now, if something like the Morganthau Plan to break Germany up into little nations had gone ahead, presumably the southern German states would revive some versions of their old, pre-Imperial flags, whereas the North, which had been dominated by Prussia, now erased from the map, would be left at loose ends and might have adopted something new and Nordic-looking, I suppose, if it didn't look too similar to the old regimes' flags. Then maybe Stalin, with the zone he controlled having little prospect of expanding to cover all of Germany, might have gone with something radical like the red banner--or maybe not, if he judged that German reunification would be in the cards eventually and he wanted to position his Germany as the leader of that movement, in which case Weimar's look is the benign alternative.
Weimar seems the most natural alternative available for anyone who wanted to reunify Germany and emphasize a break with the Imperial and Nazi programs.
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Well, here it is.
Honestly, Nanwe, if you wanted people to pay attention to the thread, why couldn't you have done that in the OP?
It's not as bad as I feared--if the yellow were white it would have been; with the yellow it's a little better. Still, it looks too Nordic, not inclusive enough of the south, to rally all of Germany. Reaffirming the legacy of Weimar seems like the smart move for both Western and Eastern factions to have made.