What is the function of the Caliph of the Muslims?

Al-Mawardi elucidated that the Caliph or Imam is obligated to fulfill ten general responsibilities: safeguarding the faith, implementing judgments, protecting the state, enforcing punishments, fortifying frontiers, engaging in jihad against those hostile to Islam, collecting spoils of war (fay’) and zakat, estimating and disbursing stipends in a timely manner, appointing trustworthy and sincere individuals, and personally overseeing affairs. If he fulfills these, he has discharged his duty to Allah, and obedience and support become incumbent upon the people.