Final answer:
The territory purchased from Russia for $7.2 million on March 30, 1867, is Alaska, known as 'Seward's Folly' until its value became evident through natural resources, primarily the Klondike Gold Rush.
Step-by-step explanation:
The territory that was purchased from Russia for $7.2 million on March 30, 1867, is Alaska. The United States, led by Secretary of State William H. Seward, acquired the Alaskan Territory, which added nearly 600,000 square miles of land to the country.
This purchase was initially met with skepticism and dubbed 'Seward's Folly' by some newspaper editors who doubted the value of the seemingly desolate land. However, it provided the U.S. with significant natural resources, including fish, furs, and, notably, the gold which sparked the Klondike Gold Rush later on.
This move significantly expanded U.S. territory and ultimately proved to be a strategic and economic boon.
In contrast, the LibreTexts excerpt incorrectly refers to the purchase of a different land area, while the other historical purchases, such as the Louisiana Purchase and the Mesilla Valley acquisition through the Gadsden Purchase, highlight the United States' expansionist history.