In 1973, the Provisional IRA extended its campaign to mainland Britain, attacking military and symbolically important targets to both increase pressure on the British government, via popular British opinion, to withdraw from Northern Ireland, and to maintain morale amongst their supporters. By 1974, mainland Britain saw an average of one attack—successful or otherwise—every three days. These attacks included five explosions which had occurred in Birmingham on 14 July, one of which had occurred at the Rotunda. Prior to any attack upon civilian targets, a code of conduct was followed in which the attacker or attackers would send an anonymous telephone warning to police, with the caller reciting a confidential code word known only to the Provisional IRA and to police, to indicate the authenticity of the threat.