What government action first allowed American businessman to come to China in large numbers

Signing of the Burlingame Treaty Apex

Explanation:The action to sign the Treaty of Burlingame on July 28, 1868 in Washington, and a year later to ratify the signing of the agreement in Beijin.

This treaty allowed both countries to maintain friendly relations of a formal nature and with multiple benefits between both nations, such as:

The right by China of Chinese citizens to immigrate freely to the United States and vice versa, as well as access to education and the maintenance of trade agreements among others.

Anson Burlingame, lawyer and representative of the United States, was the manager of this treaty with the Chinese government, hence the origin of the treaty name.