He later joined with Nguyễn Khánh to stage a successful January 1964 coup. In the next few months, the Catholic Khiêm fell out with Khánh whom he accused of being too heavily influenced by Buddhist activists. Khiêm tried to plot against Khánh, but was thwarted. He was implicated in the organization of the September 1964 coup attempt by Generals Lâm Văn Phát and Dương Văn Đức, both Catholics, and was sent into exile to serve as Ambassador to the United States. In February 1965, Colonel Phạm Ngọc Thảo, who had accompanied him to Washington had returned to Saigon and launched a coup with Phát. This was done with Khiêm's support. The coup failed but other officers took the opportunity to force Khánh into exile. Khiêm would return to Vietnam when the political climate among the generals was more favorable and became Prime Minister under President Nguyễn Văn Thiệu, resigning only in the last month before the fall of Saigon.