Lake Michigan has the largest surface area of any lake located entirely within one country.
Its surface covers about 22,400 square miles, all within the United States. That makes it larger than many countries and gives it the distinction of being the biggest lake whose entire water surface belongs to one nation.
Lake Michigan is not the largest Great Lake overall by surface area; Lake Superior is larger, but it is shared by the United States and Canada. This distinction between total size and single-country location is the key to the question.
The lake’s shoreline supports major cities and ports, including Chicago, Milwaukee, and Green Bay. Its waters connect with Lake Huron through the Straits of Mackinac, creating an unusual hydrological relationship between two separately named lakes.