With the British about to leave, it became imperative for the Jews to gain control of the territory that had been assigned to the Jewish State. Such a move would enable the Jews to have a foothold when the Arabs would invade.
To this end, an operational plan ("Plan D") was devised. This plan would be carried out in stages which would be adapted to the manner and rate of the British troop withdrawal.
The first objective of Plan D was to open the road to Jerusalem, to this end Operation Nachshon was devised. The name "Operation Nachshon" was derived from the biblical personage Nachshon Ben Aminadav who was the first to jump into the Red Sea when the Jews fled Egypt. Operation Nachshon was a first in many respects. It was the first major Haganah operation and it was the first time that a "brigade force" was employed. Before this the Haganah had operated in company-size only. The brigade force, comprised of three battalions and numbering 1,500 men, was and specifically organized for this operation. These men were armed with Czech weapons that had been smuggled into the country on April 1 at a hidden airstrip in the south. These weapons were covertly issued to Haganah members who were desperate for arms.
Two significant operations preceded Operation Nachshon: the first took place in Ramle. The Haganah blew up the headquarters of Hassan Salame (the commander of the Mufti's Army of Salvation). This attack prevented Salame's forces from thwarting Haganah preparations on the coastal plain. In the second operation, Haganah forces took over the village of Castel, an Arab village which stood between Jerusalem and Kyriat Anavim and blocked the entrance to Jerusalem.
Operation Nachshon itself began on April 6 in the Latrun area with Haganah forces taking over the Wadi al-Sarrar camp, Arab Hulda and Deir Muheisin. Parallel to this, the village of Beit Machsir in the region of Bab el Wad was attacked by Palmach forces, thus clearing the mountain road to Jerusalem. Sixty Palmach trucks drove up to Jerusalem carrying supplies. On 7 and 8 April Arab forces undertook reprisal operations, primarily in the area of Motza. The battle for Castel was fierce. For six days the Arab and Jewish forces battled fiercely. The Jewish forces were pushed back.
In a dramatic reverse, Abd el Kader el-Husseini, who had returned from Damascus to fight, was killed in battle. This proved to be the undoing of the Arab forces on April 10 immediately thereafter retreated. Supplies could now reach Jerusalem safely and by April 20, five convoys of reserves and additional troops reached the city. This condition however, lasted only a short period of time. Immediately thereafter supplies could no longer easily reach the city and convoys had to turn back, as the road to Jerusalem became impassable once again. This precipitated the siege of Jerusalem.
Sources: Israel Defense Forces