The invasion of Normandy, or more commonly known as "D-Day", is when the allied forces invaded France. It ended up with the allies fighting the German/Axis forces all the way to Germany. Pretty much like with all wars, this effected us negatively
The Soviet invasion of Afghanistan was a proxy war fought between the Soviets, and the US-sponsored Afghan militants; such as the Mujahideen. It was significant because it ultimately bankrupted the Soviets, leading to their collapse. The war effected us negatively.
The collapse of the Soviet Union was important because it essentially ended the Cold War, between the US and the USSR, with the USSR as one of the top (ex)superpowers of its time.
The sinking of the Lusitania was important because it was the reason why the US went to war in WWI; an important factor to consider when pondering our nation's history.
The treaty of Versailles was the official treaty made between the central and allied powers after WWI. It was significant because it was the end of the first World War, and arguably the beginning of the Second World War, given the extremely unfair bias there was against Germany in it; overall, it effected everybody on this planet negatively.
The New Deal was what (arguably) helped get the US back on its feet during the Great Depression - so it effected us positively. This is significant because the then repaired economy would of enabled the country to do the things that they did with the newfound economic power caused by the New Deal.
The Dust Bowl was a man-made disaster caused by too much farming. It created widespread sickness and poverty, thus making the Great Depression worse than it already was.
The Berlin wall was a wall constructed after WWII by the Soviets; it's important because it was a landmark sign of the beginning of the Cold War between the US and the Soviets; it effected us, and everybody else, negatively.
The assassination of Franz Ferdinand was what started WWI, by triggering the Austro-Hungarians into invading Serbia. It effected everybody negatively.
Hitler's rise to power was what more or less changed the direction history was taking during the twentieth century; it effected us negatively since we went to war over it.