String was selected to serve as acting Legal Adviser of the Department of State in 2019, succeeding Jennifer Gillian Newstead. As Acting Legal Adviser from 2019-2021, he served as the chief legal officer for the U.S. Department of State and directed the Office of the Legal Adviser's 375 attorneys and staff. He played a central role in the bilateral claims settlement agreement between the United States and Sudan, under which Sudan agreed to pay $335 in compensation to resolve terrorism-related claims brought by the U.S. families of victims of the 1998 bombings of the U.S. embassies in Kenya and Tanzania and the 2000 attack on the U.S.S. Cole. The claims agreement was part of a broader diplomatic initiative to restore Sudan's sovereign immunity under the Sudan Claims Resolution Act and establish diplomatic relations between Sudan and Israel under the U.S.-negotiated Abraham Accords, the normalization agreements between Israel and various Arab states.