Final answer:
The Supreme Court ruling that first reinstated the death penalty for federal criminal cases was Gregg v. Georgia (1976), which upheld the constitutionality of Georgia's new death penalty statute.
Step-by-step explanation:
The Supreme Court ruling that first reinstated the death penalty as a possible punishment for federal criminal cases was Gregg v. Georgia (1976). In this case, the Supreme Court upheld the constitutionality of Georgia's new death penalty statute, which it determined adequately narrowed the class of defendants eligible for the death penalty. This ruling ended the nationwide de facto moratorium on executions that had been in place since the Court's decision in Furman v. Georgia (1972).