By using tactical nuclear weapons there is a risk of escalating the conflict until it reaches a tipping point which provokes the use of strategic nuclear weapons such as ICBMs. Additionally, the tactical nuclear weapons most likely to be used first (i.e., the smallest, low-yield weapons such as nuclear artillery dating from the 1960s) have usually been under less stringent political control at times of military combat crises than strategic weapons. Early Permissive Action Links could be as simple as a mechanical combination lock. If a relatively junior officer in control of a small tactical nuclear weapon (e.g., the M29 Davy Crockett) were in imminent danger of being overwhelmed by enemy forces, he could request permission to fire it and due to decentralised control of warhead authorization, his request might quickly be granted during a crisis.