"A caliphate (in Arabic: خلافة khilāfa, meaning "succession") is an Islamic state led by a supreme religious and political leader known as a caliph – i.e. "successor" – to Muhammad. The succession of Muslim empires that have existed in the Muslim world are usually described as "caliphates". Conceptually, a caliphate represents a sovereign polity (state) of the entire Muslim faithful (the Ummah, i.e. a sovereign nation state) ruled by a single caliph under the Constitution of Medina and Islamic law (sharia)."
Basically, one's status as a caliph is dependent on the acceptance of that status by some group of Muslims. This follows the general pattern of religio/political formation through consensual agreement.in Islamic and Arab culture. This applies in; ideas, law, government, etc.
The declaration of the caliphate of Abu Bakr al-Baghdadi (Caliph Ibrahim) is a declaration of war against each and every existing Arab and Muslim government because in the presence of a supposed caliph all their governments must be viewed as illegitimate usurpations of the caliph's divinely given religious and political authority.