World War I (WWI), which was predominantly called the World War or the Great War from its occurrence until 1939 (World War II), and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. It involved all the world's great powers, which were assembled in two opposing alliances: the Allies (based on the Triple Entente of the United Kingdom, France and Russia) and the Central Powers (originally centred around the Triple Alliance of Germany, Austria-Hungary and Italy; but, as Austria–Hungary had taken the offensive against the agreement, Italy did not enter into the war). These alliances both reorganised (Italy fought for the Allies), and expanded as more nations entered the war. Ultimately more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. More than 9 million combatants were killed, largely because of enormous increases in lethality of weapons, thanks to new technology, without corresponding improvements in protection or mobility. It was the sixth-deadliest conflict in world history, subsequently paving the way for various political changes such as revolutions in the nations involved.

Mexican Americans are Americans of Mexican descent. As of July 2009, Mexican Americans make up 10.3% of the United States' population with over 31,689,000 Americans listed as of Mexican ancestry. Mexican Americans comprise 66% of all Hispanics and Latinos in the United States. The United States is home to the second largest Mexican community in the world second only to Mexico itself comprising nearly 22% of the entire Mexican origin population of the world. Canada is a distant third with a Mexican origin population of 37,000 as of 2001 although increasing to 61,505 as of 2006. In addition, as of 2008 there were approximately 7,000,000 undocumented Mexicans living in the United States which if included in the count would increase the US share to over 28% of the world's Mexican origin population (Note that some of the undocumented would be captured in the US Census count depending on their willingness to provide information). Over 60% of all Mexican Americans reside in the states of California and Texas. Most Mexican Americans are the descendants of the Indigenous peoples of Mexico and/or Europeans, especially Spaniards.

The Russian Civil War (25 October 1917 – October 1922) was a multi-party war in the former Russian Empire fought between the Bolshevik Red Army and the White Army, the loosely allied anti-Bolshevik forces. Many foreign armies warred against the Red Army, notably the Allied Forces and the pro-German armies. The Red Army defeated the White Armed Forces of South Russia in Ukraine and the army led by Aleksandr Kolchak in Siberia in 1919. The remains of the White forces commanded by Pyotr Nikolayevich Wrangel were beaten in the Crimea and were evacuated in the autumn of 1920. A number of independent countries – Finland, Estonia, Latvia, Lithuania, and Poland – emerged from the war.

After the abdication of Tsar Nicholas II of Russia, the Russian Provisional Government was established during the February Revolution of 1917.

In the wake of the October Revolution, the old Russian Imperial Army had been demobilized; the volunteer-based Red Guard was the Bolsheviks' main military force, augmented by an armed military component of the Cheka, the Bolshevik state security apparatus. In January, after significant reverses in combat, War Commissar Leon Trotsky headed the reorganization of the Red Guard into a Workers' and Peasants' Red Army, in order to create a more professional fighting force. Political commissars were appointed to each unit of the army to maintain morale and ensure loyalty.