Many radical Muslims believe a khalifah will unite all Islamic lands and people and subjugate the rest of the world.    From Wordnet, Princeton University. 

In the low country, where the pepper-planters reside, the title of kalippah prevails; which is a corruption of the Arabic word khalifah, signifying a vicegerent.    From Wordnik.com.  [The History of Sumatra Containing An Account Of The Government, Laws, Customs And Manners Of The Native Inhabitants] Reference

Muslims talk about the sense of humanity and creation, the khalifah principle.    From Wordnik.com.  [Christian Muslim Forum - Faith in Finance seminar at St Ethelburga's centre for Reconciliation and Peace] Reference

And this guy is a professor of Islamic law and can't get the meaning of khalifah right.    From Wordnik.com.  [Saturday, April 26, 2008] Reference

We are dealing with a radical ideology right now spreading all throughout the world with one goal, and that is the establishing of an Islamic khalifah throughout the world.    From Wordnik.com.  [CNN Transcript Nov 29, 2007] Reference

Sungguh tragis tercabut nyawa sang khalifah dalam gulungan karpet merah darahnya tidak menitik di lantai runtuhlah tahta kekhalifahan negara ummah bercempera masing-masing dengan kuasa akhirnya ummah lemah tak bermaya di hina serata dunia tragedi karpet! tregedi karpet! khalifah dalam gulungan siapa yang mahu menangis?.    From Wordnik.com.  [Planet Malaysia] Reference

The reasoning is as follows: The fulifilment of all trusts and duties is an obligation which cannot be realised except with an Imam/khalifah (which would require the necessary systems, etc.) hence because of the principle: ma la yatimmu 'l-wajib illa bihi fa huwa wajib (' that which is essential for the completion of a wajib itself becomes a wajib ') the imposition of a khalifah becomes an obligation (meaning the system, i.e. the khilafah).    From Wordnik.com.  [Khilafah.com - Building a global movement for Khilafah] Reference