In June 1961, Mandela sent a letter to South African newspapers warning the government that a campaign of sabotage would be launched unless the government agreed to call for a national constitutional convention. Beginning on 16 December 1961, the campaign by uMkhonto we Sizwe with Mandela as its leader, was launched, with bomb attacks on government targets and planned for possible guerrilla warfare. The first target of the campaign was an electricity sub-station. uMkhonto we Sizwe undertook other acts of sabotage in the next eighteen months. The government alleged more acts of sabotage had been carried out and at the Rivonia Trial the accused would be charged with 193 acts of sabotage in total. The sabotage included attacks on government posts, machines and power facilities, and crop burning.