I agree with <PERSON>' brilliant analysis and conclusion. Preventing WWII from happening is key, as well as preventing the rise of the Soviet Union. The simplest way to start is to prevent WWI.
The post-WWI economic and political landscape in Germany was such that it was bound to produce a charismatic leader that would unite the Germans and rally them to become a great power once again. Although history decided on <PERSON>, I'm sure there were many others that were standing in line ready to play that role. The anti-Semitic sentiment being what it was in Europe at the time would have been used to further gather followers (much as some politicians use popular stance on immigration to further their campaigns). This gradually increasing anti-Semitic attitude drove many of the leading theoretical physicists and scientists of the time, a remarkable number of which are of Jewish descent, to leave Germany to what would be Allied countries during WWII. As WWII broke out, the wartime atmosphere led both the Axis and the Allies to pool the finest scientific minds they could and invest heavily in science and technology. We hear about geniuses such as <PERSON>, <PERSON>, <PERSON>, <PERSON>, <PERSON>, <PERSON>, <PERSON>, and many others being brought together in Allied countries. This facilitated collaboration between these geniuses and as a result, a lot of technologies, prototypes, and theories were developed at an unprecedented rate. The foundation of many different areas of study were laid during WWII. 714
The rise of the USSR only made America double-down on military technology and scientific investment, which created the environment for the development of more modern technologies during the Cold War with the Soviet Union.
Although scientific advancement can't be entirely prevented, the acceleration of technological development is directly linked to WWII and the Cold War. Since WWI set the stage for WWII, preventing it would slow technological progress significantly. As well, if countries have other things to worry about than war, they won't typically allocate as many resources to scientific achievement as they would otherwise in a state of defense.
As I've studied the state of the world just prior to WWI, it seems conflict was unavoidable. The various political alliances and anti-government sentiments in certain countries make it a rat's nest of causes and effects. However, there are a few small things that may have changed the outcome:
* Stop <PERSON> from assassinating <PERSON>. This was the event that led to the July Ultimatum that Serbia rejected, causing Austria-Hungary to declare war. This may not help too much since there were others involved in the plot and possibly more waiting in case this failed.
* Convince <PERSON> to listen to other war advisors not to enter WWI and give up Serbia completely, hindering the involvement of more countries in the war.
* Convince <PERSON> to learn from the events leading to the 1905 revolution, stopping the momentum that the Bolsheviks used in 1917 and hindering the creation of the Soviet Union.
* Convince the rulers of Austria-Hungary to remove Point 6 from their July Ultimatum, which states that Austro-Hungarian troops would suppress "subversive movements". This was the one point that the Serbian government would not accede to.
Admittedly, these events may not prevent the war. It was a volatile time in history and events may have unfolded in a similar fashion with different actors. However, this does seem like a short list of somewhat achievable tasks. 714