Final answer:
America joined WWI due to Germany's unrestricted submarine warfare targeting American and Allied ships, sinking of American ships, and the Zimmerman Telegram, all of which led to American public opinion and congressional voices calling for military intervention.
Step-by-step explanation:
The actions of Germany before the United States joined World War I present clear arguments as to why America felt compelled to enter the conflict. Germany's unrestricted submarine warfare aimed at cutting off British and French access to American supplies, the sinking of American ships like the Laconia, and the release of the Zimmerman Telegram heightened the pressures on President Woodrow Wilson to declare war.
When the German U-boats began targeting all Atlantic traffic, including unarmed American freighters, there was a significant threat to the international concept of freedom of the seas and the rights of neutral nations. The German decision to risk American wrath was based on their desperate need to force Britain out of the war before Germany itself would have to surrender due to its dire economic situation worsened by the British blockade. Germany's gamble underscored their belief that by severing the Allies' supply lines from the US, they could secure a quick victory.
However, with the sinking of multiple American vessels and the provocation of the Zimmerman Telegram proposing an alliance against the United States, American public opinion swayed towards military intervention. This sentiment, coupled with Germany's underestimation of the speed and impact of American military mobilization, led to the US Congress declaring war in April 1917. The immediate implementation of a convoy system by the US Navy proved effective against German U-boats, ensuring the safe passage of American troops and supplies to the Allies and significantly diminishing the success of the German naval strategy.