There was opposition to World War I before we joined the war and also during it. Before the war, Americans were shocked that the Europeans went war. We thought they were very unwise in going to war. Our people wanted to stay out of the war. We knew the war would be a very costly one in terms of lives and money. German-Americans were also afraid of what they would face if we went to war. They expected they would face discrimination and possible violence if we joined World War I.
During the war, there was opposition to some of the policies that went into effect in our country. People were concerned that their civil liberties were being curtailed. The Sedition Act made it illegal to publicly criticize the President or the government. The Espionage Act made anti-war activities illegal. During a war, there is usually a conflict between protecting civil liberties and fighting a war.
There was some opposition to joining the war, and also, opposition existed during the time when we were involved in it.