What made US join in WW1?

Several factors. The primary reason was Germany's use of submarines to attack ships in the Atlantic Ocean. After a British cruise ship, the Lusitania, was sunk in 1915 by a German sub and many Americans were killed, Germany agreed to stop using submarines so much. But as the war dragged on and Britain's huge navy was making it so Germany couldn't get supplies from other countries, Germany decided they needed to start up again.

But Germany knew that the US would be angry about the submarines, so just before they restarted it, they tried to send a secret message to the Mexican government asking Mexico to fight a war against the US so the US couldn't fight Germany. British spies were able to catch the message and showed it to the US government. Right afterward, Germany restarted their submarine program. So the US was double angry at Germany and declared war shortly thereafter.