1. A computer-implemented method, comprising: adding one or more computer systems to a set of computer systems, the one or more computer systems controlled by one or more third parties to a computing resource service provider; receiving a first application programming interface request from a customer of the computing resource service provider; and fulfilling the first application programming interface request by at least: selecting a target computer system from the set of computer systems, the target computer system selected based at least in part on the target computer system being operable to instantiate a secure execution environment, the target computer system selected from the one or more computer systems controlled by a third party of the one or more third parties; sending a provisioning request to the target computer system, the provisioning request causing the target computer system to instantiate the secure execution environment on the target computer system by at least: causing the target computer system to increase an available computer resource capacity based at least in part on the available computer resource capacity; increasing the available computer resource capacity of the target computer system; and causing the target computer system to instantiate an agent within the secure execution environment using a secure execution environment key; and upon validating the secure execution environment using one or more first cryptographic measurements calculated by a processor of the target computer system, instantiating the application within the secure execution environment, the application providing one or more computer system resources.