- Violation of Belgian Neutrality: Germany's invasion of neutral Belgium, in contravention of treaties it had signed, sparked outrage in the United States. Many Americans saw this as a clear violation of international law and a threat to global stability.
- Unrestricted Submarine Warfare: Germany's resumption of unrestricted submarine warfare, which resulted in the sinking of passenger ships like the Lusitania and the deaths of American citizens, further heightened tensions between the US and Germany.
Economically:
- Trade Disruptions: The war disrupted US trade with Europe, particularly with Germany and its allies, leading to economic losses for American businesses.
- Allied Debts: The US had extensive economic ties with the Allied powers, particularly Great Britain and France, who owed significant debts to the US from pre-war loans. There were concerns that an Allied defeat could jeopardize the repayment of these debts.
- Market Opportunities: Some American industries, such as munitions and shipping, saw increased demand and profits due to the war.
Politically:
- Zimmerman Telegram: The interception and disclosure of the Zimmermann Telegram, a secret message from Germany to Mexico proposing an alliance against the US, fueled public anger and increased support for intervention.
Legal:
The President at the time, Woodrow Wilson, used the concept of "freedom of the seas" as a justification for entering the war. US Merchant ships had been attacked and sunk by German submarines (U-boats), and the US government felt it had a legal right to defend American lives and property.
To summarize, the US entry into World War I can be seen as morally justified based on the violation of Belgian Neutrality and the loss of American lives due to unrestricted submarine warfare, economically justified by the disruption of trade and the desire to protect Allied debts, politically justified by events such as the Zimmermann Telegram, and legally justified based on the defense of American lives and property.