No country attacked the US before they joined WW1, but the US's allies such as France and Britain were attacked and at war with Germany, so the US decided to support their allies and join the war. The sinking of the British/ American ship "The Lusitania" was a trigger for the US to join the war.
The first country the USA attacked during WW2 was the country that had attacked them, Japan. First action against Japan was the doolittle raid on April 18 1942 where a few American bombers bombed Tokyo.
We got rich
The US was first attacked at Pearl Harbor by the Japanese.
Chemical weapons were first used in war by Germany in WW1.Nuclear weapons were first used in war by the US in WW2.
Great Britain was attacked by Germany and was aided by the US in its resistance.
Libya attacked our US Embassy which is US territory
Marines
The U.S. got involved because Britain was being attacked by the Central Powers (enemies) and Britain is allies with the U.S..
Japan
Japan
Japan.