The British had a pact with the Belgians to protect them in the event of a major conflict. Once the Germans began blazing a path to France through the small country, Britain's hand was forced and their entrance into the first major global conflict became unavoidable.

The attacks that launched the War on Terror were the 9/11 attacks coordinated by Osama Bin Laden and the Taliban. These attacks occurred on September 11, 2001. None of the other mentioned attacks led to the officially named War on Terror; however, the attacks of Pearl Harbor did lead to American involvement in World War II.

"Trinity" was the code name of the nuclear atomic bomb tested in Nevada as part of the Manhattan Project, the American development of the bomb.

The Mujahideen were Afghan resistance fighters who organized the Afghan defense forces during the Soviet Invasion of Afghanistan. Many countries provided aid to the Mujahideen including Pakistan, the United States, and China. Much of the support and weaponry given to the Mujahideen during the Soviet Invasion of Afghanistan has since been used on the army of the United States of America during the American occupation of Afghanistan.

Americans contributed to the war effort in many ways; however, the internet was not available at this time so emails could not be sent to soldiers.

The dates of the Korean War are: June 25, 1950 to July 27, 1953. The Korean War was between North and South Korea but had great international intervention. The United Nations, particularly the United States, fought for the South while China and the Soviet Union fought with the North. Though the causes are complex, it arose from tensions caused by WWII and the Cold War.

The Russo-Japanese War is often seen as a turning point in history because it was the first time a modern eastern power had defeated a western power in open warfare. The previous centuries had been marked by one victory after another for the west, but with victory in the Russo-Japanese War this trend was halted.

The USSR (known today as Russia) was the Communist leader during the Cold War was the world was split between communism and free enterprise ideas.

The correct answer is the Japanese aerial attack on Pearl Harbor, Hawaii. The United States had remained neutral, aiming to avoid entering another war, until Japan conducted these aerial strikes in 1941.

"German U-boats sinking Lusitania" is an incorrect answer because that event, though it did influence the United States' involvement, occurred during the First World War.

The Bombing of Britain did influence some United States' decision to enter because of being close allies; however, the US remained neutral throughout these desperate times in the UK.

Germany invaded Poland in 1939, but none of the Allies aided Poland in any military or aid way despite declaring war on Germany because of this action.

The Russian Revolution in 1917 had many complex causes, some were deeply-rooted in society at the time whereas others were more immediate. Succession of wars, the rise of the working class, mass disapproval of the Czar, and issues facing peasants all contributed to the outbreak of the revolution; however, Czarism ended because of this revolution and wasn't a cause. The other answers are all direct causes to the growing discontentment of the Russian people that prompted a revolution.