In 1776, with the Signing of the Declaration of Independence, the former Thirteen Colonies of the British Empire declared their independence as the United States of America. The British Empire saw this as little more than a rebellion, and entered combat with the Americans to keep them in their place. This ignorance ended in 1783, when the American Revolution ended in a British surrender, allowing the United States of America to successfully become a sovereign nation.
After the success of the American Revolution, the new United States of America adopted the idea of "Manifest Destiny" and started to expand west, towards the Pacific Ocean, but this was temporarily stopped in 1861, when the United States of America ended its state of peace and fell into disarray when the American Civil War started between the United States of America and the Confederate States of America, a collection of states in the southern US who hoped to secede. The American Civil War ended in 1865 with a victory for the United States, successfully quelling the civil war.
Things were normal in the United States for a few decades while it expanded west until it reached the Pacific Ocean and even expanding farther than the coast by annexing the Kingdom of Hawaii and some other small island nations. This era of (mostly) peace ended in 1917 when the United States of America joined the Allied powers in the First World War (which had been fought since 1914) when the German Empire attacked American ships in the Atlantic. The First World War ended a year later with the Treaty of Versailles.
After the First World War, the United States wasn't done with war. In 1941, the Japanese Empire coordinated and executed an attack on the United States Navy in Pearl Harbor, Hawaii. This attack drew the United States into the Second World War against Nazi Germany and the Japanese Empire(which had been raging since 1939.) This Second World War ended in 1945 when the United States dropped two nuclear warheads over Hiroshima and Nagasaki, Japan, causing an immediate surrender and an end to the war.
With the axis power defeated and the world seemingly at peace, the United States found a new enemy in the Soviet Union, another nation with a power to rival the United States. The United States of America entered into a Cold War with the Soviet Union, this war never became hot, though, other than the proxy wars fought in Korea and Vietnam, the United States and the Soviet Union never enters and open fighting. The Cold War, as it became called, included many tactics to belittle the enemy nation, including propaganda, threats, and epic feats, including the first manned landing on the Moon. The Cold War ended in 1991 when the Soviet Union collapsed and all states became independent republics.
On September 11, 2001, an event which would forever change the world was executed. The 9/11 terrorist attacks were followed by complete Western intervention in the Middle East, freeing nations from terrorist threats and instability. Most of these nations went on to form the United Arabian Federation in 2017. The War on Terror reached it height when in 2015, a terrorist group got hold of a crude, but still effective, nuclear warhead and launched it towards Washington, DC. This warhead was destroyed just after it reached space and a United States responded by dropping a Nuclear Warhead, dubbed Zahhak, was dropped in Southern Iran at the site of the terrorist's Capital City and Launch Site. After the Arabian Missile Crisis, the Western powers spearheading the War on Terror began to use more effective tactics to wipe out the terrorist groups once and for all. The International War on Terror ended in 2017 when all major terrorist groups in the middle east were wiped out.
After the International War on Terror ended, the United States of America was finally able to turn its eyes away from the middle east and to the stars. With the formation of the United Nations Space Administration by the United States, Russia, Republic of China, France, Germany and Great Britain, Humanity was finally able to land humans on Mars. This historic mission started the Second Space Age, which stands until this day [2147 CE,] propelling humanity to the stars.
In 2024, increased hostilities between the United States and Mexico boiled over and caused Mexico to declare war on the United States. This was an uphill battle and eventually the United States captured Mexico City and annexed everything north of it. Just a year later, Canada joined the United States. Shortly after this, many small Caribbean nations joined the United States as somewhat autonomous states in order to stay alive after a huge economic depression in the Caribbean. After this, in 2029, What was left of Mexico, Belize, Guatemala, El Salvador, and Honduras join the United States after successful referendums.
Today, the United States is a powerhouse, both economically and politically. With a flourishing economy and friendly relations with almost every other political power in the solar system. The United States of America is truly one great nation out of many.