June 10, 1915 Romania declares war on Russia joining the Central Powers. With the threat of war with Japan the German Empire signed an agreement with Australia transferring control of German New Guinea to that country and it sold the Northern Marianas islands to the United States in a secret agreement.
On June 11,1915 Japan declared war on Germany, Austria-Hungary and Italy. On June 17 it landed troops in China and the Northern Marianas Islands. The United States Informed the Japanese Government that the Islands were US territory and to remove its forces from the islands. On June 18 the Republic of China demanded the removal of Japanese troops from its territory and when the Japanese refused it declared war on Japan on June 19,1915.
On June 18,1915 the government of Great Britain informed the Japanese Empire that the alliance between the two nations was now terminated.