Complex sentence: During this conference, King Abdul-Aziz had gained a legal fatwa to carry on a war against the Sharifs of Mecca to ensure the freedom of performing the Hajj for the people of Najd.
Split into simpler sentences: During this conference, King Abdul-Aziz had gained religious legal authority, or fatwa, to carry out a war against the Sharifs of Mecca. This is to ensure the freedom of performing the Hajj for the people of Najd.