The Reign of Terror began in 1793, when Maximillien Robespierre helped form the Committee of Public Safety and its associated Revolutionary Tribunal, which was empowered to try and execute the "enemies of the revolution." These enemies proved to be almost everyone not associated with Robespierre's Jacobin parties, leading to widespread anti-Jacobin sentiment among the masses. From the anger towards the Terror, the public wanted wholesale changes, which helped lead to the rise of Napoleon Bonaparte.

The assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian Empire, in Sarajevo on June 28, 1914 by Serbian nationalists was the immediate cause of World War I. How this conflict spread throughout the European continent was through a series of longstanding and complex alliances between various powers. When Austria-Hungary declared war on Serbia, Russia's alliance with Serbia meant it had to declare war on Austria-Hungary, followed by Germany declaring war on Russia, France declaring war on Germany, and then Britain declaring War on Germany.

During the early years of World War II, the Allies, and in particularly Britain, started to run low on supplies, but unfortunately most of Europe was under Nazi control and could not be counted on to trade. At this same time, the United States was attempting to remain neutral in the war, as many people were in favor of isolationism. Eventually the war became bad enough that it appeared as though the United States would have to pick sides, even without fighting directly. To avoid direct conflict, the Lend-Lease Act was passed in 1941, which allowed the government to give military supplies to foreign nations that were allies of the United States. This act made it so that the debts did not have to be paid back immediately, although typically the United States was given military bases in these countries in exchange for supplies. The act also essentially severed trade ties with members of the Axis Powers, as the U.S. began to help Britain, France, the Soviet Union, and other countries.

The trials of Nazi war criminals were conducted in Nuremberg, Germany due to the fact that the Palace of Justice located there was undamaged and provided enough space needed to detain the prisoners and hold trials. The trials started with the most significant war criminals in 1945, and those trials lasted almost a year. Of the most significant tried, twenty-four people were found guilty by an international tribunal. The rest of the trials were carried out from 1946 to 1949 by the U.S. military.

After World War I, France was concerned about Germany becoming militarized again and invading France. In an attempt to preempt this, France developed a protective barrier along the German and Swiss borders. The Maginot Line was built between 1929 and 1938 with some of the best military technology of the time and seemed impenetrable. Unfortunately for the French, they decided not to build the Maginot Line at full strength by the Belgian border, as Belgium was a country that remained neutral, and France did not want to do anything that could bring conflict into Belgium. The Germans realized this and invaded France through Belgium, rendering the Maginot Line useless to stop an outside invasion. While the Line initially seemed like a deterrent to invasion, the decision not to fortify it fully was what ultimately lead to France’s downfall in World War II.

The War of the Spanish Succession began in 1701 over who would rule over the Spanish Empire after the death of Charles II. The empire that the Habsburgs had built up extended all over Europe and was of great interest to many European leaders. After some attempts to achieve a practical solution, Charles II declared that Philip, Duke of Anjou, a grandson of the King of France, would become the next emperor. This would have given France a tremendous amount of power, and this worried many countries, particularly England, the Netherlands, and Austria, who worked to counteract this expansion of French power. The ensuing conflicts lasted until 1714, when a variety of treaties, collectively called the Peace of Utrecht, were reached that broke down to most of the empire being divided up among the European powers and Philip ruling Spain after agreeing not to seek the French crown.

Following the end of World War I, much of Europe lay in ruins, and most of the European governments did not have the money or the resources to rebuild. The United States realized that it needed to take a lead in rebuilding Europe, not only to help the countries, but also to lower the chances that communism would take hold in Western European countries. Some of the Eastern European countries had already succumbed to communism, and the USSR was in a position to expand its influence. The plan was named after the Secretary of State, George Marshall, and was implemented in 1948 with support from Congress and President Truman. The plan, which spent billions of dollars helping to rebuild Europe and its economy, was quite successful and is credited with helping Europe to recover. It also showed the disadvantages of communism when the USSR refused American aid to help East Germany and other countries within the Soviet Bloc.

The War of the Austrian Succession began in 1740 when Frederick the Great invaded Austria-controlled Silesia under the pretense of Maria Theresa not being a lawful heir to the Austrian throne, which she obtained following the death of her father Charles VI in that same year.

When France ceded Canada to Britain in 1763, the colonists living there became British subjects. The Quebec Act of 1774 was meant to appease them and win over their loyalty to Britain, not deport them from the country. The incredibly sparse colonial population of Canada at the time necessitated measures to keep subjects in Canada, not expel them.

The Thirty Years' War occurred in the 17th century, lasting from 1618 to 1648. The other three conflicts spanned the 18th century: War of the Spanish Succession (1701-1714), War of the Austrian Succession (1740-1748), and Seven Years' War (1754-1763).