Once the Americans defeated Spain in the Spanish-American war and gained much of Spain's former colonies in the Pacific, Spain's Empire was crushed.

The "domino theory" refers to the belief that if one country adopts communism (or "falls" to it), then its neighboring countries will "fall" too. It was used to justify intervening in the fight between Communist North Vietnam and non-Communist South Vietnam. 

Kaiser Wilhelm II was the last German emperor, and was deposed in 1918 after a 30 year reign. 

Mutually Assured Destruction refers to the prevalent philosophy during the Cold War that if either the USSR or the USA launched a nuclear attack, the other could retaliate with sufficient speed and power that both would be destroyed. This kept the Cold War from erupting into nuclear conflict.

During World War I, huge losses were incurred by combatants when they sent unarmored soldiers against machine guns.

The Russian Revolution was bloody, but grew out of World War I.

The Chinese Civil War was bloody, but by that time military planners already knew how to deal tactically with machine-guns.

By the time of the Battle of Midway, war planners already knew the effectiveness and danger of machine-guns.

By the time of the Persian Gulf war, war planners were using global positioning systems (GPS) and air strikes, machine-guns were already well understood.

The Chinese Civil War lasted almost half a century, in the middle of which the Japanese invaded.

After the Japanese invasion, many factions within the Chinese civil war joined forces against the invaders, but fighting resumed once the Japanese had been defeated.

Although the Japanese invasion certainly changed the dynamic of China's Civil War, the war began before the Japanese invasion.

Some factions within the Chinese Civil War were anti-communist, while others were supported by Moscow and pro-communist.

When Archduke Franz-Ferdinand of the Austria-Hungary Empire visited the Bosnian capital of Sarajevo, he was assassinated by Serbian nationalist Gavrillo Princip. As a result, Germany aligned with Austria-Hungary against Serbia (and it's major ally Russia), and soon nations were taking sides and war broke out. This assassination was the spark that ignited the growing nationalist and militaristic tendencies that had been growing throughout the end of the 19th century.  

Advances in technology led to a completely new style of warfare that came with the turn of the 20th century. While hand to hand combat fell out of style, armies would dig trenches to protect their front lines, which they would defend with Machine guns and heavy artillery. The result were vast stretches of "no man's land" between trenches that were virtually deserted for fear of being gunned down. These mass killing technologies led to significantly higher death rates.

WWI brought the concept of Total War to new heights, in this global war the conflict stretched far beyond the military. Factories were re-purposed for the war effort, propaganda was spread, and food was rationed. In many nations conscription was established to recruit more actively for the armed forces. 

The Treaty of Versailles placed almost total blame for WWI on Germany and imposed major punishments on them including: stripping them of domestic and oversees territories, limiting the size of the military, imposing harsh economic sanctions. Resentment born from this treaty would eventually cause Hitler's rise to power and World War II.