1. The Third Reich Complete Their V3 Superweapon
It's a good thing that Hitler's secret V3 superweapon never saw completion. This massive multi-charge cannon would have been capable of indefinitely raining down one shell a minute onto London or any other southern British location. Thankfully, Allied forces came across it when storming through northern France, at a point where it was only a few months away from completion. Its completion may not have won the war for the Nazis, but it would have completely devastated postwar Britain's economy, claiming hundreds of thousands of civilian lives along with it.
2. Franz Ferdinand Escapes Assassination
The assassination of Archduke Franz Ferdinand in Sarajevo in June 1914 is considered one of the most influential murders of all time. Without Ferdinand's assassination, World War I would probably never have started, making Hitler's election, and the start of World War II, highly unlikely. Experts believe that without WWI, the Russian Revolution might not have succeeded, which would have lead to European powers retaining global dominance instead of America.
3. Carlos II Produces an Heir
The Spanish War of Succession was a turbulent period of 13 years that resulted in the balance of power shifting from Spanish and French control to British dominance. Due to multiple generations of inbreeding, Carlos II was unable to produce a healthy heir, and this is one of the primary reasons behind such a devastating war. If Carlos II had actually become a father, then this war would never have taken place, and it's quite possible that Spanish would be the world's most common language.
4. Bill Clinton's Secretary of Defense Tells Him the Truth About N. Korea
In 1994, Bill Clinton asked Secretary of Defense William Perry whether they could take out North Korea's reactor at Yongbyon without causing contamination. Even though he believed that it was possible, he decided to tell the president that it wasn't to prevent starting another war. Had he told him the truth, the US may have been involved in another massive war, which experts estimate could have killed more American soldiers than the Vietnam war. On the other hand, we wouldn't have to deal with Kim Jong-Un today, though.
5. The Romans Win the Battle of Teutoburg Forest
The Battle of Teutoburg Forest was an incredibly important moment in Roman history. After the Germanic tribes wiped out around 10% of the Roman army, they vowed never to return to northern Europe again. This boundary is what protected much of the diversity we can see in the world today. Had the Romans conquered northern Europe as well, English and other Germanic languages would have ceased to exist, and most of the world would have become Roman Catholic, thus altering art, science, and culture in unimaginable ways.
6. Pearl Harbor Never Gets Bombed
Despite what many people might think, had Japan not bombed Pearl Harbor, the US would probably still have entered WWII. The Japanese were so desperate for resources that they would have ended up attacking other US territories anyway, meaning that America would have eventually joined at a much later date. Experts believe that if America had left it a lot longer to join WWII, then Stalin may have tried to conquer most of Europe in the meantime.
7. Napoleon Never Invades Russia
In what has been claimed to be the most foolish mistake a military commander ever made, Napoleon's invasion of Russia left his army humiliated and decimated, leading to his ultimate defeat. If he hadn't made this terrible mistake, it's highly likely that Napoleon would have ended up molding the whole of Europe in his image. The strangest part is that Napoleon only invaded Russia to 'punish' Alexander I for trading with the British, and not for any real strategic reason.
8. Rommel Is Present During D-Day
One of the most influential Third Reich commanders, Erwin Rommel was unfortunately away celebrating his wife's birthday when the Allies annihilated his army during D-Day. Had he been present, D-Day might have ended far differently, since the Germans' lack of a commander left then panicked and confused. If the Allies had been repelled, then the Nazis may have had enough time to complete their V3 superweapon, thus decimating London and many of its civilians.
9. Marie Antoinette and Louis XVI Survive the French Revolution
The French Revolution completely changed the way the world was perceived, as many old certainties were shattered, and conditions were put into place to prevent ancient empires from forming again. All of this was possible thanks to Louis XVI's decapitation at the guillotine. However, Louis XVI very nearly did escape to the royalist alliance. He had arranged for a light carriage to take him to his allies as swiftly as possible, however, at the last minute, Marie Antoinette insisted on going with him. This meant that they had to take a larger, and thus slower, carriage, which wasn't fast enough to get them to safety in time.
10. 9/11 Is Stopped
The multiple intelligence failings that allowed 9/11 to take place have been heavily documented, and many experts believe that it could never have been prevented. However, if the plot had somehow been foiled, Islamist terrorism wouldn't be such a rampant issue today, since 9/11 ended up inspiring countless others to carry out similar acts around the world. Mass surveillance would also be far less widespread, while American culture would be even more powerful and dominant than it is today.