This was the nuclear weapon used by the U.S. to force Japan to surrender during WWII.

The United States detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki in August 1945, killing 210,000 people—children, women, and men. President Truman authorized the use of the atom bombs in an effort to bring about Japan's surrender in the Second World War.

I believe the answer is: It viewed European interference in the Americas as a threat to US national interests.

It helped colonies in North and South America adopt a democratic form of government.

The monroe doctrine view European interference in America as a form of manifestation of an unfriendly disposition toward the United States.

He believed that all the 'help' that the European give is a camouflage for their true intention, which is wakening United States' government from the position of Ally and slowly taking over control and replace our democracy.