Pacifism encourages individuals and groups to refuse to take up arms to defend their country (or an ally) against aggression. As such it weakens the ability of the country to defend itself and therefore threatens the security of the country or the alliance to which it belongs.
Pacifist demonstrations in the West have been overtly encouraged by the USSR and its allies, to the USSR's advantage. No pacifist demonstrations occurred in the Eastern bloc when the USSR deployed SS-20s. Pacifists should recognize the pernicious effects of pacifism in some circumstances, for example in the demobilization that occurred before the second world war. During the Cold War, Europe was under threat and the balance of power was no longer stable; confidence cannot be re-established simply by political will.
Military force never solves problems. Pacifists look back on a tradition in the West from the early Christians who refused military service, to the Society of Friends (the Quakers), and quote Ballou, Thoreau, Tolstoy, Aldous Huxley, Schweitzer and many others who shared a 'reverence for life'. In the East, pacifism has been preached from the time of Buddha to that of Gandhi.