Lebanon was split between the Christians and the Muslims, the Muslims wanting to side with the USSR and the Christians wanting to side with the US. The president, Camille Chamoun, was Christian and when the Muslims revolted, he called to the US which was being led by President Eisenhower. The USSR threatened to start a nuclear war if the US helped Lebanon, but the US did so anyway, calling Khruchev's bluff. The US then effectively restored power to Chamoun.
The tension between the Christians and Muslims intensified and lead to the Lebanese Civil War from 1975-1990. The Muslims were aided by the PLO, but Syria sent troops to control land there. Many factions were destroyed after Israel and the US wiped out PLO presence; the country was war-torn.
The overall instability of the country was due to Cold War aftermath. Several groups and countries fought for control, such as the Muslims and the Christians, the partial Syrian occupation, and the attempted control from Israel and the US. All this left Lebanon in fragile condition.
US-Lebanese relations were also affected. President Eisenhower sent the Marines to Lebanon to prevent the overthrowal of the Christian government. Later, during the Arab-Israeli conflict, their alliance with the US discredited them as a country which caused Anti-American feelings to spread.
Finally, Lebanon's economy never truly recovered. The past few years have seen Lebanon dependent on other countries like Saudi Arabia. The immigration of Syrian refugees has also caused an economic decline.