Sink the Titanic?

[SuedodeuS]

If this is something like Centrum vs Homeline, you really just need to have Centrum trying to save the Titanic. As soon as Infinity gets wind of this, they'll be wanting to insure the Titanic sinks after all - they don't want Centrum's dastardly plot (that would presumably shift the timeline closer to Centrum) to succeed. That's really all the motivation you need.

Now, if you want to figure out the reason behind why Centrum is trying to save the Titanic, many of the suggestions in this thread are excellent. I particularly like the ones where a key figure (or two) who died instead survives and causes significant change in the timeline. In order to prevent the characters from just offing the individuals who would cause the change, you make certain they don't know who Centrum is trying to save - or at least that there's the possibility that they don't know everyone that Centrum is trying to save.

[DocRailgun]

I know this doesn't really answer the OPs question, but...

an interesting mission might be for the PCs to take something that Man Was Not Meant To Know (like the Necronomicon) or the Spear of Destiny to a disaster like the Titanic or the Hindenberg and make sure that the disaster happens to destroy/hide forever the dreaded artifact.

Of course, the last scene could be the characters reading about James Cameron's attempts to salvage items around the wreck.

<cue dramatic music>

[RyanW]

Originally Posted by DocRailgun

an interesting mission might be for the PCs to take something that Man Was Not Meant To Know (like the Necronomicon) or the Spear of Destiny to a disaster like the Titanic or the Hindenberg and make sure that the disaster happens to destroy/hide forever the dreaded artifact.

Or, if you know when a specific container is recovered, you could put a message in said container...

"Scooty Puff Jr. sucks!"

[madickey]

John Jacob Astor was transporting plans for a new invention made by fellow inventor Nikola Tesla to America for further testing and to bring to prototype: a time machine! Unfortunately, Astor has familiarized himself with the plans well enough that either his survival or the discovery of the plans by his cousin William Waldorf Astor would cause time travel to come a full century (or whatever) before OTL, and easily fall into the wrong hands.

[Irish Wolf]

Originally Posted by DocRailgun

(A)n interesting mission might be for the PCs to take something that Man Was Not Meant To Know (like the Necronomicon) or the Spear of Destiny to like the Titanic or the Hindenberg and make sure that the disaster happens to destroy/hide forever the dreaded artifact.

Of course, the last scene could be the characters reading about James Cameron's attempts to salvage items around the wreck.

<cue dramatic music>

The artifact is already on board - the PCs don't know where, but in the Original Timeline, the artifact was used the following day, causing something horrible like sunken Rl'yeh's rise from the ocean depths. They have to stop that, and if they can't find the artifact in time, their superiors have notified them that sinking the ship/crashing the airship with all hands is an acceptable alternative...

[Mylon]

Originally Posted by Turhan's Bey Company

I
However, close cooperation between government and an oligarchy of tycoons soon turns matters for the worse...

This. The successful voyage heralds a stronger era of corporations than we know today due to the industrialist determination it symbolizes, regardless of anyone in particular on board, though Bey here has provided a wonderful amount of detail. Other possibilities (aside from WW3) include massive environmental failure from over exploitation, a much greater industrialist project gone wrong (Tower of babel?), slavery of the common people to the wealthy executive.