This bill addresses the construction of a border barrier system along the southern border, such as requiring construction activities to resume if certain conditions are met. Specifically, during any period in which the Armed Forces or the National Guard are performing active service at the U.S. southern land border, the Department of Homeland Security (DHS) must resume all activities related to constructing physical barriers and accompanying infrastructure that were underway or planned prior to January 20, 2021. DHS must also (1) expend all funds appropriated or explicitly authorized for constructing such a border barrier system on or after October 1, 2016; and (2) honor all agreements related to the construction of the border barrier system that were entered into before January 20, 2021, with the consent of all applicable parties to revive such agreements. DHS may not cancel any contracts for the construction of the border barrier system entered into on or before January 20, 2021. The bill also extends the availability of certain funds made available from FY2018 through FY2021 for construction of the border barrier system.