Final answer:
Kennedy singled out NATO as the group of countries that the United States is committed to defending. Established in 1949, its principle of collective defense means that an attack on one member is considered an attack on all, forming a key military alliance during the Cold War.
Step-by-step explanation:
In response to the student's question, the group of countries that President Kennedy proclaimed the United States would defend against any attacker is the North Atlantic Treaty Organization, commonly known as NATO. Established in 1949, NATO's core principle of collective defense is enshrined in Article 5 of its treaty, which states that an attack on one member is considered an attack on all. This alliance, which initially consisted of the United States, Canada, and ten Western European nations, significantly expanded its membership over the years, solidifying its role as a central pillar of Western military cooperation and a deterrent against potential Soviet aggression during the Cold War era.