After the First World War broke out, the United States adopted a policy of neutrality at first. The public opinion was not in a mood favorable to participation in a distant war fought by selfish European powers. So, president Woodrow Wilson would have had a very hard time trying to get Congress pass a declaration of war in 1914. It was only after the sinking of the British liner Lusitania in May 1915 by a German U-boat and the loss of 128 American lives that Americans started to change their mood and supported the Allied cause against Germany. The US finally entered WWI in 1917.
to impose a tax on items by requiring them to purchase a stamp also it was a tax to help pay for the British troops stationed in the colonies during the 7 year war