It's "I'll Be Seeing You" by Sammy Fain (music) and Irving Kahal (words), written in 1938 but emblematic of WWII and the painful separations of that time. Mark Steyn has a fascinating discussion of the song and its enduring appeal:
As much as "It Had To Be You" or "The Way You Look Tonight", "I'll Be Seeing You" belongs to a select group of über-standards, the ones we'll still be singing when 90 per cent of the rest have fallen away.Mark does a little geometry on the various recordings Sinatra made of it:
Which is the real Sinatra "I'll Be Seeing You"? Both. There's no correct way to do the number. The definition of a standard is a song you can do in a zillion different ways - and sometimes with the same singer, and all in the same year. Grab a piece of paper and draw a triangle. Point A is Frank's 1940 record of "I'll Be Seeing You". Point B is Sy Oliver's May 1961 swinger. Point C is Axel Stordahl's September 1961 ballad version. The lines between A and B and between A and C mark Sinatra's artistic growth, and the line between B and C marks his emotional range.Here's the 1961 Stordahl ballad version recorded for Point of No Return. Very lovely. But it will come as no surprise to the three or four people interested in this list that I prefer Sy Oliver's exuberantly swinging take from I Remember Tommy:
Bob Belvedere ranks it at #36:
Francis recorded several versions of this Great American Songbook tune [see Mark Steyn’s top-notch take on this wonderful tune here], but whereas the others are more serious, I find this celebratory one much more satisfying. I like sad, but not dispirited. Life goes on, pal.Frank swings it mightily but somehow -- as usual -- still retains the emotional content of the song.