"All quiet along the Potomac": A phrase used by Northern newspapers in the weeks after the Union defeat at Bull Run, making fun of General McClellan's interminable delay in attacking the Rebel forces. This phrase also became the name of a popular Union song.
I'm curious about your statement that newspapers were using the phrase to make fun of McClellan. In fact, I'm not sure if it was known or used much at all in newspapers, until after the famous poem/song, and there it wasn't making fun of McClellan so much as pointing out the tragedy of individual deaths.
Do you have some examples of it from period newspapers, making fun of McClellan, to support that? Or, for that matter, has anyone identified the newspaper that Ethel Beers claimed had the famous headline?
http://en.wikisource.org/wiki/Page:All_quiet_along_the_Potomac_and_other_poems.djvu/356 I tried looking at fultonhistory.com which has a lot of New York state newspapers, but couldn't find anything that seemed to fit, but of course they don't have everything, by far.
Also, I don't think the song was really considered to be a Union-only song. It seems to have been adopted both north and south. For example, the
Southern Literary Messenger claimed it had been "published in all our papers" and attributed authorship--wrongly, as they later admitted--to Lamar Fontaine of the 2nd Virginia Cavalry.
Here's sheet music published in South Carolina. And
here's an 1864 diary entry, p. 152, from Louisiana: "she sang some for us, 'Lorena,' ... and 'All quiet along the Potomac tonight,' were the songs she sang, all beautiful." The lyrics also show up
here, in a booklet of "Lays of the South," published in England to raise money to help Confederate prisoners. In other words, with numerous southern examples, I don't think it was a popular
Union song, so much as a popular song in general.