While today it seems like the USA maintains a diplomatic presence in nearly every country, that was not always so. How did the State department prioritize back when it had a smaller budget? What were the old pre-cold war US embassies like?
I can comment to a limited extent on US diplomatic presence in the Middle East. My understanding is that prior to early-to-mid 20th century it was deeply connected to religious and educational institutions and often quite regional. So the US had a consul in Lebanon in connection with American University in Beirut (until 1920 the "Syrian Protestant College") as well as a consul in Jerusalem.
Worth noting here are terminologies like "Regional Security Officer", which still exists as a position in all US embassies but as the name suggests was formerly a region-wide position.
Of course, it helped in the Middle East that there were far fewer countries to deal with before the dissolution of the Ottoman Empire. Likewise, there was less need for consular services before the modern passport/visa systems were put in place.