United Nations Secretary-General Antonio Guterres on Friday urged de-escalation of the conflict between Israel and Iran because "we are on course to chaos."

Leaders from the U.K., Russia, China, Germany and beyond have weighed in on the growing conflict, which is showing no signs of de-escalating.Urgent international calls for restraint appear to be falling on deaf ears as the world braces for intensifying conflict in the Middle East.

Israel and Iran have been enemies for decades — but this is their most sustained direct fighting ever. Here’s how a shadow war erupted into an overt conflict.