Complex sentence: On 12 March 1984, Scargill declared the NUM's support for the regional strikes in Yorkshire and Scotland, and called for action from NUM members in all other areas but decided not to hold a nationwide vote which was used by his opponents to delegitimise the strike.
Split into simpler sentences: On 12 March 1984, Scargill declared the NUM's support for the regional strikes in Yorkshire and Scotland, and called for action from NUM members in all other areas. However he decided not to hold a nationwide vote, which became an argument used by his opponents to delegitimize the strike.