When Israel achieved its independence on May 14, 1948, the Haganah became the de facto Israeli army. On that day, the country was invaded by the regular forces of Egypt, Lebanon, Iraq, and Syria. Eleven days later, Israel's provisional government issued an order that provided the legal framework for the country's armed forces. The order established the official name Zvah Haganah Le Yisrael and outlawed the existence of any other military force within Israel.

The dissident Irgun and Stern Gang were reluctant to disband. Fighting between Irgun and regular military forces broke out on June 21 when the supply ship Altalena arrived at Tel Aviv with 900 men and a load of arms and ammunition for the Irgun. The army sank the ship, destroying the arms, and many members of the Irgun were arrested; both organizations disbanded shortly thereafter. A more delicate problem was how to disband the Palmach, which had become an elite military unit within the Haganah and had strong political ties to the socialist-oriented kibbutzim. Nonetheless, David Ben-Gurion, Israel's first prime minister and minister of defense, was determined to see the IDF develop into a single, professional, and nonpolitical national armed force. It was only through his skill and determination that the Palmach was peacefully abolished and integrated into the IDF in January 1949.

The ranks of the IDF swelled rapidly to about 100,000 at the height of the War of Independence. Nearly all able-bodied men, plus many women, were recruited; thousands of foreign volunteers, mostly veterans of World War II, also came to the aid of Israel. The newly independent state rapidly mobilized to meet the Arab invaders; by July 1948, the Israelis had set up an air force, a navy, and a tank battalion. Weapons and ammunition were procured abroad, primarily from Czechoslovakia. Three B-17 bombers were bought in the United States through black market channels, and shortly after one of them bombed Cairo in July 1948, the Israelis were able to establish air supremacy. Subsequent victories came in rapid succession on all three fronts. The Arab states negotiated separate armistice agreements. Egypt was the first to sign (February 1949), followed by Lebanon (March), Transjordan (April), and finally Syria (July). Iraq simply withdrew its forces without signing an agreement. As a result of the war, Israel considerably expanded its territory beyond the United Nations (UN) partition plan for Palestine at the expense of its Arab neighbors. Victory cost more than 6,000 Israeli lives, however, which represented approximately 1 percent of the population. After the armistice, wartime recruits were rapidly demobilized, and the hastily raised IDF, still lacking a permanent institutional basis, experienced mass resignations from its war-weary officer corps. This process underscored the basic manpower problem of a small population faced with the need to mobilize a sizable army during a wartime emergency. In 1949, after study of the Swiss reservist system, Israel introduced a three-tiered system based on a small standing officer corps, universal conscription, and a large pool of well-trained reservists that could be rapidly mobilized.