In return, Japan allowed Thailand to invade and annex the Shan States and Kayah State in northern Burma, and resume sovereignty over the sultanates of northern Malaya which had been surrendered in the Anglo-Siamese Treaty of 1909 with Britain. In January 1942 Phibun declared war on Britain and the United States, though the Thai Ambassador to the United States, Seni Pramoj, refused to deliver it to the state department. Seni denounced the Phibun regime as illegal and formed a Seri Thai Movement in Washington. Pridi, now serving in the role of an almost powerless regent, led the resistance movement inside Thailand, while former Queen Ramphaiphanni was the nominal head in Great Britain.