1920s: POSTWAR BOOM
Prior to World War I, and well into the 1920s, ship decor relied on period styles. And with period styles, came very little in terms of original art. As you'll see above, this would shift in the latter part of the decade, particularly with the Kungsholm and the Bremen. (The Ile de France will be added later.)
After the war, the surviving passenger ships were pressed back into service as quickly as possible, with a focus on conversion from coal to oil as fuel. Interiors were largely kept intact, even with Germany's fleet divvied up among the winners. As it turned out, those who lost ships, received big reparations from Germany. Cunard's loss of Lusitania gave them Imperator, which was renamed Berengaria. The sinking of White Star's Britannic resulted in Majestic, the former Bismarck.
The United States didn't really have a notable passenger fleet, as we were still a country growing from within. Most ocean traffic was generated by people emigrating to America. Still, mostly by luck, as the ship was stranded at its New York pier when we finally entered the war, the United States ended up with the giant Vaterland, renamed Leviathan while being converted to a troopship, plus a patchwork quilt of smaller German liners, which formed the basis of the United States Lines.
Two classes of troopships, the 535s and 505s, completed too late for World War I, were used by American companies until World War II. Our first true newbuilds didn't happen until later in the decade. We start with Matson's Malolo, and end with Panama Pacific's California, Virginia, and Pennsylvania.