The original agreement where the national government gained possession of Fort Sumter and Fort Moultrie was executed in 1805 and reads in part:
"That, if the United States shall not, within three years from the passing of this act, and notification thereof by the governor of this State to the Executive of the United States, repair the fortifications now existing thereon, or build such other forts or fortifications as may be deemed most expedient by the Executive of the United States on the same, and keep a garrison or garrisons therein, in such case this grant or cession shall be void and of no effect."
The fortifications had not been repaired by April 1861, much less in 3 years. Indeed a full 56 years had passed between the signing of the lease and the events of April, 1861. Fort Sumter had been empty until it was garrisoned December 26, 1860 when Major Robert Anderson moved his troops from Fort Moultrie to Fort Sumter in response to South Carolina's secession; this FIRST act of the so-called Civil War was done even though there was an existing agreement in place between the federal government and the government of South Carolina leaving Anderson AT MOULTRIE without interference by EITHER side.
With regard to Sumter, the United States had failed to fulfill it's responsibilities in the lease agreement with South Carolina and that fort had legally reverted to South Carolina. Hence, not only did Anderson destroy parts of Fort Moultrie, but he and his troops illegally invaded and occupied territory that had reverted back to South Carolina.