Before the United States has entered the Great War under President Woodrow Wilson, Germany disrupted the American trade with other countries, an example was the sinking of the Lusitania. The Lusitania was a passenger ship but since Germany did not want America to have any trade with Europe, Great Britain and France, America could not stop the business interest, so the United States used a passenger ship to ship war materials and food supplies. But during May 1915, the Germany U-boats found out and sunk the Lusitania, 1300 people were killed and 128 of then were American. The sinking of the Lusitania was one of the main reasons that the United States entered the war. During the war, Germany declared war on the Allied powers. Germany had lost trade with the United States, but Germany has developed new war weapons such as mustard gas, gas poison and trenches. As soon as Germany did, the allied powers developed their technology too, cars, planes and tanks. The trade disruption by Germany has let the United States enter the war, the United States have lost American soldiers and allied power friends, but the United States Navy became the world's greatest Navy and the growth of the United States government has proved to the whole world that the United States has the power to lead.
In 1910, the United States declared war on Spain, which created the Spanish-American War. This gave Mexico a bad impact of the United States. So not long after the Germany U-boats sunk the "passenger ship" Lusitania, they sent the Zimmerman Telegram to Mexico, hope they could join the central powers and get back on America. According to President Woodrow Wilson, the Zimmerman Telegram violated the Monroe Doctrine. And from the Monroe Doctrine, if other countries have come into the Western Hemisphere and want to start war. America will use military power to solve the problem. This is also why the United States declared war on the central powers. As the United States entered the war, Germany developed war weapons and the United States lost about 13 million soldiers and 20 million was wounded. Including medical, war materials, the development of technology and lost of business interest in Europe the United States has lost over $300 billion dollars. The United States government used Liberty bonds to maintain the United States economic, about all citizens loaned money to the government. The United States is developing, but we have lost soldiers, economic interest and money.
The Zimmerman Telegram has violated the Monroe Doctrine; the sinking of the Lusitania has shut down all trade between Europe and the United States. The Great war taught the US a lesson. But the United States proved that we have the power to be a leader.