Most important events in American History

I'm not sure that this qualifies as a significant event but in the mid 1700's much of the United States was owned by France.
Please refer to the map below.

Source:
I find the details of Treaty of Paris to be very interesting. It was done in part to end the seven years war with the French and the natives which had help from Spain.
Exchange of territories

During the war, Britain had conquered the French colonies of Canada, Guadeloupe, Martinique, Saint Lucia, Dominica, Grenada, Saint Vincent and the Grenadines, and Tobago, the French "factories" (trading posts) in India, the slave-trading station at Gorée, the Sénégal River and its settlements, and the Spanish colonies of Manila (in the Philippines) and Havana (in Cuba). France had captured Minorca and British trading posts in Sumatra, while Spain had captured the border fortress of Almeida in Portugal, and Colonia del Sacramento in South America. In the treaty, most of these territories were restored to their original owners. Britain however made considerable gains.[3]
France and Spain restored all their conquests to Britain and Portugal. Britain restored Manila and Havana to Spain, and Guadeloupe, Martinique, Saint Lucia, Gorée, and the Indian factories to France. In return, France ceded Canada, Dominica, Grenada, Saint Vincent and the Grenadines, and Tobago to Britain. France also ceded the eastern half of French Louisiana to Britain; that is, the area from the Mississippi River to the Appalachian Mountains.[4]
Spain ceded Florida to Britain. France had already secretly given Louisiana to Spain in the Treaty of Fontainebleau (1762). In addition, while France regained its factories in India, France recognized British clients as the rulers of key Indian native states, and pledged not to send troops to Bengal. Britain agreed to demolish its fortifications in British Honduras (now Belize), but retained a logwood-cutting colony there. Britain confirmed the right of its new subjects to practice the Catholic religion.[5]

France ceded almost all of its territory in mainland North America, but retained fishing rights off Newfoundland and the two small islands of Saint Pierre and Miquelon, where it could dry that fish. In turn France gained the return of its sugar colony, Guadeloupe, which it considered more valuable than Canada.[6]

Source:

Louis XV preferred to keep La Guadeloupe which was known for its spices rather than keep the territories in La Nouvelle France.
It is rumoured that he is quoted as saying: " Je ne me batterai pas pour des bancs de neige."
Translated in English, it means: "I will not fight for snow banks."
Here is what I believe to be a good source but it is in French.

Then in 1803 there was the Louisiana Purchase.

France controlled this vast area from 1699 until 1762, the year it gave the territory to its ally Spain. Under Napoleon Bonaparte, France took back the territory in 1800 in the hope of building an empire in North America. A slave revolt in Haiti and an impending war with England, however, led France to abandon these plans and sell the entire territory to the United States who had originally intended only to seek the purchase of New Orleans and its adjacent lands.
The purchase of the territory of Louisiana took place during the presidency of Thomas Jefferson. At the time, the purchase faced domestic opposition because it was thought that it was unconstitutional. Although he agreed that the U.S. Constitution did not contain provisions for acquiring territory, Jefferson decided to go ahead with the purchase anyway in order to remove France's presence in the region and to protect both U.S. trade access to the port of New Orleans and free passage on the Mississippi River.
Source: The above map is the modern day United States with the Louisiana Purchase overlay (in green).

I sometimes try to imagine what would have happened if Louis XV would have preferred to keep La Nouvelle France and trade off La Guadeloupe instead.
I also try to imagine how things would be today if the Louisiana Purchase did not happen.