In 1870 he was a member of the commission sent by President Grant to inquire into the practicability of the annexation of Santo Domingo. President Grant wished to annex the island. He was opposed in this effort by Sen.Charles Sumner, a longtime friend and ally of Howe's. In the end, the committee sided with Sumner in opposition to the proposed annexation. Grant was so enraged at having his plans thwarted that he arranged to have Sumner removed from his chairmanship as head of the Senate Foreign Relations Committee.