Less well known today was his "documentary" on the sinking of the Luisitania. The ship was blasted in 1915 by a German submarine, and it helped propel the United States to enter World War I. The sinking outraged McCay (and many other Americans) and he spent his free time developing this twelve minute film, often touted as the longest animated work at the time.
As no one who was on hand survived, McCay primarily relied on the account of August F. Beach, a reporter based in Berlin who worked for William Randolph Hearst. Beach was the first reporter to get on the original story. The film is pretty clearly a propaganda piece but it's starkness was highly unusual for animation (it was still considered very much a medium of, at best, brief diversion) and McCay's abilities were so far beyond his contemporaries that no one could attempt anything comparable for years afterwards.
The film took nearly two years to complete, with McCay painting the majority of the cells himself. Unlike his previous forays into animation, however, The Sinking of the Luisitania did poorly at the box office. But even coupled with Hearst's less than subtle attempts to force McCay into doing comic strips exclusively, McCay went on to produce six more films by 1921. Luisitania seems to me as one of his more personal and heartfelt films, though.
Take a look for yourself...