what event or person do you think changed History in the 20th or 21st century? Give 5 or more reasons?

Answer 1

After considering people such as Hitler (he changed the world, but for how long?) and events such as the Versailles Treaty at the end of WW I, I finally settled on the atomic power and the bomb, it's development and it's use or threatened use.

First, the creation of atomic weapons and also nuclear power is what could change the world from using fossil fuels to using energy that is entirely renewable.
Second, the bomb ended WW II
Third, the bomb became the backbone of the Cold War ... it was the fear of total annihilation that kept the two sides, East and West within the bounds of reason ability and not in an all out war.
Fourth, it was the fear of nuclear war that ultimately led to the end of the Cold War and the fall of the Soviet Union. It just wasn't reasonable to be spending the kind of money that both the USA and USSR were spending, to maintain that edge. Sooner or later something had to give.
Fifth, nuclear power today holds the key to renewable and long lasting energy sources so that we can stop burning and using oil, changing the entire politics of the middle east.

Answer 2

The terrorist attacks of Sept 11, 2001...

Reasons? Many individuals, groups, and governments have radically changed their habits, choices, opinions, laws, regulations, and other requirements as they reacted/responded to these events.

The world is very different now than it was before the towers fell...

Answer 3

einestein...........relativety theories,both the general and special as well.
it affected ALL fields,really all fields