U.S. officials warned Tuesday that a cyberattack could be harmful enough to lead to a real war, a reaction to the hacking attempt against military contractor Lockheed Martin Corp. Some experts, however, say there are “real challenges” to linking a cyberattack to a specific group and pursuing retaliation—especially against foreign governments. Several former and current national security officials said the Pentagon did not appear overly concerned about the cyberthreat until after the Lockheed breach. Since then, the Defense Department said about 100 foreign intelligence organizations try to break into U.S. networks each year. Critics of the U.S. say China and Russia are better equipped nations.