In fact, apart from the United States, there was another country that also wanted to join the Allies in declaring war on Germany, and that was the Island Nation.
However, the Island Nation was different from the United States. It had neither an advanced economic system like the United States nor a navy strong enough to be taken seriously by Britain and France.
Moreover, the Island Nation had a severe shortage of mineral resources, which joining the Allies would require assistance from Britain and France.
Therefore, Britain and France directly refused the request of the Island Nation to join the Allies, and the Island Nation's plot to seize Germany's colonies in East Asia during the war was also thwarted.