When world War Two broke out, he volunteered for the Commandos in the British Army and was decorated for his heroism in battle against the Italians in Abyssinia.
After his demobilization, he took part in Irgun military operations. He was appointed commander of the Fighting Force in Petah Tikva, and took part in a large number of actions. Among other things, he was in charge of the attack on the Lydda Military airfield and the raid on the railway station and bridges at Ashdod. Shimshon also expanded the Irgun's training program, and introduced the rifle and Bren gun into operational use.
In all the operations he commanded, he went into battle at the head of his troops. This was true in the case of the Acre jailbreak as well. Bren gun in hand to cover his comrades should they encounter a British ambush, he was killed in just such an encounter, aged 32.