From 1914 to 1916, as world war i raged in europe, americans were not able to remain neutral in thought as well as action mainly because

The freedom of the seas was interfered with by the war in Europe.

The British had imposed a naval blockade against Germany, and Germany responded with submarine warfare against shipping coming into Britain.  The most famous instance -- which almost pulled the USA into the war at that point -- was the sinking of the passenger ship Lusitania by a German torpedo in May, 1915. Over a thousand people drowned when the ship sank, including 128 American citizens. Germany suspected that munitions were being shipped into Britain in the cargo holds of passenger liners.  That was shown after the fact to have been true in the case of the Lusitania.