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Sentry, known for most of its lifetime as LoADS for Low Altitude Defense System, was a short-range anti-ballistic missile (ABM) design made by the US Army during the 1970s. It was proposed as a defensive weapon that would be used in concert with the MX missile, a US Air Force ICBM that was under development.
The LoADS concept was one of a number of proposals that were made as part of a larger and acrimonious debate over the best way to base the MX missile. It was believed that by about 1980 Soviet ICBMs would improve to the point where it was possible to attack US ICBMs while they were still in their silos. A number of warfighting scenarios suggested that a surprise attack would significantly reduce the US stockpile and greatly blunt any counterattack. In the case of the MX, a single successful strike on a silo would mean ten warheads would not reach the USSR, making these extremely valuable targets. To ensure such an attack would fail, at least to the point where the Mutual Assured Destruction (MAD) would be maintained, a wide variety of basing options were considered that would guarantee the survival of at least dozens of MX's.
One way to improve the survival of the ICBM force would be to actively defend it with an ABM system. However, the 1972 ABM treaty greatly limited the number and geographical deployment of any ABM, with the aim of preventing whole-country protection and thus ensuring MAD. LoADS addressed these limitations by being deployed along with the radars and engagement computers needed to make a successful attack at only very short ranges, 50,000 feet (15 km) or less. The entire system was packed into a cylinder that looked like an MX and would be shuffled between the silos. As the LoADS could be in any of the silos at a particular missile site, the enemy would have to expend two warheads on every silo to ensure a hit, as one would be assumed lost to LoADS. This would not stop a successful counterforce attack, but it would make it significantly more expensive in terms of the number of warheads used, potentially requiring more warheads than the Soviets had.
With President Jimmy Carter's decision to base the MX in a series of less-hardened horizontal silos in 1977, LoADS development was accelerated. The system as a whole was renamed Sentry, with the missile becoming the Baseline Terminal Defense System, or BTDS. Work on MX and Sentry was further accelerated by incoming President Ronald Reagan in 1981. However, after another review of the MX program, Reagan chose a different MX deployment concept and cancelled Sentry, stating that it would violate the ABM treaty. As the questionable security of the MX once again became an issue, Reagan briefly entertained an even shorter-range system known as Swarmjet, before the entire MX program was severely curtailed with the ending of the Cold War.

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