 Beginning in 1998 and continuing through 2009, the Propulsion Replacement Program extends the life and maintains the performance by replacing the old solid propellant boosters (downstages).The Single Reentry Vehicle (SRV) modification enabled the United States ICBM force to abide by the now-vacated START II treaty requirements by reconfiguring Minuteman-III missiles from three reentry vehicles down to one.Though it was eventually ratified by both parties, START II never entered into force and was essentially superseded by follow-on agreements such as SORT and New START, which do not limit MIRV capability.