They declared war on Germany's German empire. US was independent and did not want to officially join the allies. Before entering the war the US remained neutral and has been an important supplier to Great Britain and allied powers. Under the leadership of Pres. Woodrow Wilson, the war represented the climax of the progressive era as it tried to bring reform ad democracy to the world, even when there was substantial public opposition to the US' entry in the war. Hope this helps!
