BERLIN — In April, the North Atlantic Treaty Organization will celebrate its 70th birthday. Founded in the earliest years of the Cold War, it is just as relevant today, when many feel that the international order is shaken again. In fact, if NATO did not exist, those in favor of a free world would have to invent it.

While NATO’s key purpose remains to guarantee the security of its members, it has never been a purely military alliance. It is a political alliance as well, based on the common aspirations of its members who, as the NATO Treaty says, “are determined to safeguard the freedom, common heritage and civilization of its peoples, founded on the principles of democracy, individual liberty and the rule of law.”

These principles are under assault today. Russian aggression in Eastern Europe, Chinese assertiveness in the South China Sea, the terrorism of the Islamic State spreading from the Middle East to the capitals of Europe, authoritarian regimes developing nuclear weapons — as different as these challenges are, they have one thread in common: They emanate from actors who oppose the international order. They try to undermine or even change the rules that have governed the age of democracy and prosperity since World War II.