A deterrent threat must be communicated clearly and understood. Misunderstandings, misperceptions and miscommunications frequently occur. In a 1950 speech, the U.S. Secretary of State omitted South Korea from a list of allies, leading the North Koreans and their Soviet and Chinese sponsors to mistakenly believe that the U.S. would not respond to an attack. During the resulting war, as American troops pushed through North Korea and approached the Chinese border, the Chinese communicated through third parties (China and the U.S. had no diplomatic relations) that they would not tolerate American troops on their border. The U.S. ignored the warning, and the Chinese intervened in massive numbers, prolonging the war for years.
In 1990, when the American ambassador to Iraq acted on orders from her superiors and told Saddam that the U.S. had no position on his dispute with Kuwait, he felt it was a green light for an invasion. (The State Department later unfairly fired the ambassador for its own mistake.)
Deterrence theory assumes that the attacker rationally understands the capability and determination of the victim to resist and retaliate. However, sometimes the danger is not clearly understood. When Argentina invaded the British-held Falkland Islands in the 1980s, it did not think that faraway Britain would have the will or capability to respond. In fact, Prime Minister Margaret Thatcher declared war, defeated Argentina, declared a special election and won a large majority.
Few in the Japanese leadership anticipated the disastrous consequences of the Pearl Harbor attack, which mobilized the U.S. to destroy Japan. Nor did Russia anticipate the fierce battle it got from tiny Finland in the Winter War of 1939-40. Greece put up such an unexpected fight against Italy in WWII that Germany had to intervene, disastrously delaying their invasion of Russia. Similarly, the U.S. underestimated the difficulties of fighting in Vietnam, Afghanistan and Iraq.
Some analysts feel that Carter’s weak response to the taking of hostages at the U.S. embassy in Iran in 1979, Reagan’s withdrawal after 241 U.S. Marines were killed in Lebanon by a Hezbollah suicide bomber in 1983, Clinton’s withdrawal after 18 U.S. deaths in Somalia in 1993 and his responding to the U.S. embassy bombings in Africa in 1994 with a missile barrage but no troops on the ground, all gave the impression that the U.S. lacked the will to respond to such attacks. So, the 2002 invasion of Afghanistan in response to 9/11 was a surprise to the Al Qaeda and Taliban leadership.