The systems represent the final chance for countries to resolve conflicts before they escalate to shooting.

When North Korea sought to show its displeasure over new UN sanctions on March 8, it went straight for the jugular.

The hot line.

Effective immediately, North Korea announced, it was severing its hot line with Seoul. And, it warned, tensions were so high that "a nuclear war may break out right now."

The warning of nuclear war was perhaps more bluster than substance. But by hanging up its hot line, Pyongyang found a way to demonstrate its anger with a gesture that anyone, anywhere, could recognize.

Hot lines are potent symbols because, around the world, they have come to represent the final chance for countries to resolve conflicts before they escalate to shooting. The imagery goes back to the Cold War days, when the prospect of a nuclear war was real enough that last chances were needed.

The most famous hot line is between Washington and Moscow. It was established in 1963, a year after the Cuban Missile Crisis. In those days, the hot line to Moscow was popularly known in America as the "red phone" and many people actually believed it was red -- the color of emergency phones.

A popular U.S. film in 1964, "Dr. Strangelove," shows the American president phoning the Soviet premier, with the main concern being whether they can hear each other:

In fact, the original hot line was not a phone at all but a teleprinter, or telex, link. Messages could be encoded and typed on one end, and decoded and read out at the other.

The coding was necessary to keep the communications private. But it also was considered to offer a better chance of reaching an understanding than if leaders spoke directly over the phone during a crisis -- when tempers might get the better of good judgment.

Since 2008, the Moscow-Washington hot line has become a secure computer link over which messages are exchanged by e-mail. Hot-line technology, like everything else, changes with the times.

Another famous hot line is the one between Moscow and Beijing. It was used during a 1969 frontier confrontation between the two countries, then severed by Beijing after it refused a Soviet peace attempt. The hot line was revived in 1996.

There are also hot lines between London and Moscow and between Paris and Moscow, both dating back to the Cold War.

More recently, NATO and the Russian military established a hot line last month.

All these hot lines are intended to provide a safety net for tussles between rivals. So, too, does the hot line between India and Pakistan. They set up their link in 2004, after both countries test exploded nuclear bombs and almost wage a new war over disputed Kashmir at the end of the 1990s.

Today, hot lines are considered such a fundamental part of international relations that even some countries which have no conflicts share them. Brunei and Vietnam, for example, have very good relations. But they, too, announced last year they would setup a hot line.

And why not? As North Korea showed on March 8, one never knows when a hot line might come in handy, or for what reason.

Already, North Korea has slammed down its hot line with Seoul on five previous occasions. Reason enough to suspect it will soon restore the line so it can do so again.