The correct statement is "Warring nations to interfere with the United States' freedom of trade."
This refers specifically to the role that Germany played in bringing the US into World War I. One of the main events that drew America's attention to this war was the sinking of the Lusitania. This British vessel, with American citizens on it, was sunk by a German U-Boat and resulted in the death of over 100 Americans.
Despite Woodrow Wilson warning the Germans about this use of unrestricted submarine warfare, the Germans used it multiple times on US ships that were trading in what was considered a war zone.
