With the dissolution of the Ottoman Empire after World War I, Syrian nationalists proclaimed their country independent, but in 1920 France was given a League of Nations mandate over it. The Druses led a futile revolt in the south in 1925–26. In World War II Syria was occupied by Allied forces; in 1946, it became independent. Meanwhile, in 1945, it had helped found the Arab League.

When Israel was created in 1948, Syrian troops were among those who fought against the new nation. Growing sentiment for Arab unity was stimulated by the existence of Israel. There were frequent armed clashes along the Syrian-Israeli border, leading in 1967 to the Six Day War between the Arab nations and Israel. Syria lost the Golan Heights, overlooking Israel. The border clashes, involving Palestinian commando groups, soon resumed. ( .)

Meanwhile, Syria's internal affairs had been chaotic. Coups toppled one government after another. The radical Baath (Arab Socialist Resurrection) party gradually became dominant and in 1958 joined Syria with Egypt in the United Arab Republic. The results displeased most Syrians and a coup in 1961 ended the union. The Baathists again rose to power, and the extremist faction of the party held control from 1966 to 1970. During this time, foreign aid was received from the Soviet Union. Palestinian commandos operating from Syria were given official support.

In Jordan, government opposition to the commandos led to civil war in 1970. Syria sent tanks and troops to aid the commandos, but its forces were quickly repulsed. Consequently, the Syrian government was overthrown and moderate Baathists, under General Hafez al-Assad, took control. He became president of Syria in 1971. Relationships with Western countries improved. A new constitution was adopted in 1973.

In 1973 Syria joined Egypt in attacking Israel in an attempt to regain territory the two countries lost in the 1967 war. The war ended with Israel still in control of the Golan Heights. In 1976 Syria sent troops into Lebanon in an attempt to end a civil war between Christian and Muslim militias. The civil war persisted, and Syrian troops remained in the country.

In 1982 an uprising in Hamah by the Muslim Brotherhood, a fundamentalist group, was crushed by government troops who killed some 10,000 residents in an assault on the city. In 1990 Syrian troops destroyed the last Christian militia in Lebanon, ending the civil war. Syrian forces continued to occupy Lebanon, to help maintain the peace. They were withdrawn in 2005. In 1991 Syria joined the coalition of forces that defeated Iraq in the Persian Gulf War.

After ruling for 30 years, President Hafez al-Assad died in 2000. His son Bashar al-Assad succeeded him as president.

In the early 2000's, many Lebanese citizens and foreign governments urged Syria to withdraw its forces from Lebanon. In February 2005, Rafik Hariri, a former prime minister of Lebanon, was killed by a car bomb in Beirut, Lebanon. Many people accused Syria of playing a role in Hariri's death, but Syria denied the charge. The assassination sparked large anti- and pro- Syrian protests in Lebanon and led to increased international demands for Syrian withdrawal. Syria completed a troop withdrawal in April 2005.