Certainly throughout the 1970s and 1980s, it was not at all uncommon to see an American synagogue where the Aron Kodesh was flanked by American and Israeli flags.
Pre-1948, was there just an American flag at the average synagogue? Or no flag at all? Or something else?
Lehavdil (about churches), a note at the bottom of this page claims:
In the United States the practice seems to have developed during the Taft administration when the world was heading toward WWI. Franklin Roosevelt issued a statement requesting churches to have the flag placed in churches as we entered WWII. My memory is that this was done reluctantly and at the advice of and pressure of Secretary of War Stimson.
So it may well be that much the same is true of synagogues.
I spoke to a person who was a teenager in the late 1930s. He said the Israeli flag was known as the Jewish flag then, and was in the Shuls even prior to 1948. As you can see from this link in Wikipedia http://en.wikipedia.org/wiki/Flag_of_Israel the flag was designed for the Zionist Movement in 1891. It became a Jewish symbol starting in late medieval Prague, and was adopted by the First Zionist Congress in 1897.
The question remains as to whether there were American flags alone in Shuls prior to this date.
I always figured that the American Flag was only put up because the synagogues wanted to put up the Israeli Flag while still showing respect to the United States.
It appears that it is not against the Flag Code to fly a foreign flag without also flying an American Flag, but it does make sense not to open yourself up to criticism about favoring a foreign country over the one you're living in.
If so, the flags only started after Israel became a state (or whenever the flag started being associated with Israel). I have no proof for this, however.
