"What is best described as a Philosophy Song, is now being sung--and whistled by the troops as they march along. We believe that it will become overwhelmingly popular." -Vide Press, 1915

Fully titled "Pack Up Your Troubles In Your Old Kit Bag and Smile, Smile, Smile!" and alternatively shortened to "Smile, Smile, Smile"; arguably the most popular tune in England during World War I.

Private Perks is a funny little codger
With a smile, a funny smile.
Five feet none, he’s an artful little dodger
With a smile, a funny smile.
Flush or broke, he’ll have his little joke,
He can’t be suppress’d.
All the other fellows have to grin
When he gets this off his chest--Hi!

By 1915, Britain had become mired in the throes of the First World War. Thousands upon thousands of troops continued to stream into France and Western Europe. By the war's end, over 900,000 British troops would be killed, and more than twice that number wounded. The losses were most painful early in the war, when the Central Powers still had momentum from their initial advances; British morale at this point was at its nadir. With little to take comfort in, the reeling British Empire found comfort from a variety of sources, but that which took the fore in boosting attitudes was a small, simple song.

Pack up your troubles in your old kit-bag,
And smile, smile, smile,
While you’ve a lucifer to light your fag,
Smile, boys, that’s the style.
What’s the use of worrying?
It never was worth while, so
Pack up your troubles in your old kit-bag,
And smile, smile, smile!
(sung twice after each verse)

George and Felix Powell were brothers, a pair of amateur musicians the likes of whom were common in the early 20th century during the gradual rise of the Victrola. They would write and compose songs to play in various English music halls; one that they penned in 1912 contained part of what would eventually become "Pack Up Your Troubles".

Little information is available concerning its creation, but it is known that the Powells shelved the song for a few years before bringing it back out again for a marching song competition in 1915. George, the lyricist, published it under the psuedonym of George Asaf, while Felix Powell was credited with the music. After their entry won, the Powell brothers began to incorporate it into their musical acts. Its popularity quickly became widespread, and hundreds of thousands of copies were soon sold around the world, with Britain being by and far its center of acclamation.

Private Perks went a-marching into Flanders
With his smile, his funny smile.
He was lov’d by the privates and commanders
For his smile, his funny smile.
When a throng of Bosches came along
With a mighty swing,
Perks yell’d out, "This little bunch is mine!
Keep your heads down, boys, and sing--Hi!"

The song remained ingrained in the minds of the masses, and so its words readily lent themselves to a plethora of references. The most popular of these was likely the 1932 movie Pack Up Your Troubles, one of the first non-silent feature films to feature the immortal comic duo of Laurel and Hardy. The plot managed to send the two comedians through boot camp and World War I, give them responsibility of a fallen war comrade's child, and take them through a series of escapades involving an attempt to return the girl to her grandfather, all in a whirlwind 68 minutes. It is generally acclaimed that Pack Up Your Troubles, while one of the less polished of Laurel and Hardy's collaborations, still is quite humorous on the whole and merits watching.

Private Perks, he came back from Bosche-shooting
With his smile, his funny smile.
Round his home he then set about recruiting
With his smile, his funny smile.
He told all his pals, the short, the tall,
What a time he’d had;
And as each enlisted like a man
Private Perks said, "Now my lad", Hi!

"Pack Up Your Troubles" (the song) continues to make its mark on the world even today. The phrase "Smile, smile, smile" has entered the general lexicon, including its becoming the title of a poem by Wilfred Owen. It has been covered by a preponderance of bands, choral groups, and other musical groups, most recently by the Canadian folk duo Sons of Maxwell--which, serendipitously enough, is actually comprised of two brothers.

Snippets, almost always of the chorus, have been featured in (among other things) an episode of the Nickelodeon television series "Rugrats", where the cowardly Chuckie used the lines to allay his fears. Overall, "Pack Up Your Troubles" still stands as a song that is remembered for, if nothing else, its boundless and unabashedly cheerful optimism.

Unfortunately, it could not give this happiness to one of its creators; Felix Powell intentionally shot and killed himself in 1942.

Pack up your troubles in your old kit-bag,
And smile, smile, smile,
While you’ve a lucifer to light your fag,
Smile, boys, that’s the style!
What’s the use of worrying?
It never was worth while, so
Pack up your troubles in your old kit-bag,
And smile, smile, smile!

Sources:
http://www.imdb.com/title/tt0023312/
http://www.usgennet.org/usa/mo/county/stlouis/ww1-music/kitbag.htm
http://ingeb.org/songs/packupyo.html
http://www.oucs.ox.ac.uk/ltg/projects/jtap/tutorials/intro/owen/smile.html
"Pack Up Your Troubles" MIDI can be found at http://www.usgennet.org/usa/mo/county/stlouis/ww1-music/kitbag.mid
All song citations are from works composed before 1923 and are thus in the public domain.

Thanks also to Stealth Munchkin and Laurel and Hardy Filmography for setting me straight on chronological issues.

node what you kind of know, but need to learn more about before you can properly node it
critical acclaim (or lack thereof) is left as an exercise for the reader