Natural shipping: Germany had a policy of unrestricted attacks on shipping vessels thought to be carrying troops, supplies or weapons for the Allies. This policy was suspended after vigorous protests by the USA because it violated American neutrality. In 1917 Germany reactivated this policy and the US finally decided to declare war on Germany in April of the same year.
Trench warfare: Trench warfare was devastating for both belligerents. The cold, the chemical weapons, the bombs, the fighting the diseases, the malnutrition, the unsanitary conditions killed millions of young soldiers. Due to the devastating loses, this generation of young victims and survivors came to be known as the Lost Generation.
U-Boats: Germany greatly developed this method of warfare and sank hundreds of shipping and military ships from Allied and neutral nations.
Liberty Bonds: these were debt securities that were sold to the general population to finance the war effort. Billions of dollars were collected thanks to the liberty bonds. There were cases of malfeasance by stock brokers in Wall Street.