The re-militarization of the Rhineland was significant because it represented a German violation of the Treaty of Versailles, which established a demilitarized Rhineland to protect France from German invasion. After this incident Hitler realized that the war weariness of the other major European powers would allow him to act without fear of repercussion. In 1938 Nazi Germany annexed Austria in what is known as the Anschluss.

The Russo-Japanese War (1904-1905) was primarily fought for control of the Korean peninsula and Manchuria. It ended in victory for the Japanese and their continued expansion into mainland Asia.

The Molotov-Ribbentrop Pact was an agreement between the Soviet Union and Nazi Germany not to attack one another. It was signed in 1939 and lasted until Nazi Germany invaded Soviet occupied Poland in 1941, thus violating the pact. The agreement also included secret provisions to divide Poland and much of Eastern Europe between the two major powers.

The Ottoman government’s stated reason for carrying out the Armenian Genocide was fear that the Armenian inhabitants of the Ottoman Empire were loyal to the Russians. This was a time of precipitous decline and great insecurity for the Ottoman Empire, which lived under constant fear of Russian invasion.

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.

The Iraqi invasion of Kuwait took place in 1990 and led to the involvement of the United States and the outbreak of the Gulf War.

At the end of the Second World War Korea was split into two occupied territories - a communist north occupied by the Soviet Union and a capitalist south occupied by the United States of America. This ultimately led to the outbreak of war in 1950. The war ended in a stalemate and the nation of Korea continues to be divided along the thirty-eighth parallel between communist North Korea and capitalist South Korea.

The three most intense phases of the Cold War (the phases when open warfare or nuclear confrontation seemed most plausible) are probably the aftermath of World War Two and the Korean War; the early 1960s and the Cuban Missile Crisis; and the 1980s. For much of the 1970s there were talks of arms reductions and a cautious detente was being reached; however, in the latter half of the 1970s these trends started to reverse as tensions increased between the two superpowers. This was primarily the result of the Vietnam War, improving relations between the United States and China, and the Soviet invasion of Afghanistan.

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 Gulf War took place from 1990 to 1991 and was triggered by Iraq's invasion of Kuwait. Iraq invaded Kuwait in August 1990 and the US, with its allies, launched Operation Desert Storm as a response, starting a war. Iraq's actions were condemned by the international communities, and economic sanctions were placed on the country. Led by the US into Saudi Arabia, many countries joined the Coalition and began aerial and ground attacks. 

The other options for this question are incorrect because they are not accurate for the time period, geographically incorrect, or irrelevant in this particular outbreak of war.