1. Time travel is possible, but under tight control in the milieus where it exists. In particular, there is a strictly enforced agreement between the various factions against assassinating historical figures. A cabal intent on delaying a mid-twentieth century advance in quantum mechanics realizes that the best way to get rid of its inventor without making the deed obvious and exposing it to undoing (in the fiction we will allow the sort of mad rewriting via repeated time traveling that Charles Stross does in “Palimpsest”) is to find one of the several averted crises of early 20th century Europe and subtly un-defuse it, thereby killing most of that generation. Whether it works as intended will be left open.
2. In the late nineteenth century the owners of the western world realize that without some kind of really significant culling, the progression of things is going to reach a critical point and a functioning version of Marx’s utopia will become inevitable. The preventative measure they have in mind is essentially a repeat of their 1871 operation, and mechanisms are set up to have a bit of a short sharp shock in 1914, lasting at most two years. Agents are put in place in the various great powers, but then most of the architects die on the Titanic and it gets a bit out of hand.