Along with his army, the Messenger of Allah always fought on the frontlines of the battle; but such was not the case for Abu Bakr, Umar, and Uthman – they never battled alongside their soldiers during their years as caliphs. Instead, they sat in Madinah and dispatched forces.
When Abu Bakr assumed the caliphate, rather than fight himself, he sent Usama b. Zayd to fight the Romans. When Usama inquired as to why Umar was remaining in Madinah, Abu Bakr said that he needed him there. When Usama then asked Abu Bakr why he himself was staying behind in Madinah, Abu Bakr told him, “Just lead the army and go.”
When the Bani Umayyah came to power, they fabricated stories saying that Ali advised Umar and Abu Bakr not to lead their armies, since they were the leaders of the people. Such stories are obviously false, since during his own caliphate, Ali himself led his troops and thus he would not have advised others to do contrary to this.
Despite the unwillingness of the first three caliphs to participate in military activity, the Muslim conquest during their caliphate was tremendous. The Islamic influence spread far and during the reign of Umar, the Persian and Roman Empires were both defeated. However, the motive of the first three caliphs to send the Muslims into battle was less for military necessity than it was to prevent political unrest at home. This ploy is most visible in the advice that Abdullah b. Aamir gave to Uthman b. al-Affan when he instructed him to keep the Muslims preoccupied in battle so that Uthman would have less difficulties with the masses.
 Tarikh al-Tabari, 2:462