 For over a year after the settlement with the Mamluks , calm settled over an @-@ Nasir Yusuf 's reign , but on 11 December 1256 he sent two envoys to the Abbasids in Baghdad seeking formal investiture from the caliph , al @-@ Musta 'sim , for his role as " Sultan " . This request was connected to an @-@ Nasir 's rivalry with Aybak , as the title would be useful in future disputes with the Mamluks . However , the Mamluks had sent their envoys to Baghdad previously to precisely ensure that an @-@ Nasir Yusuf would not gain the title , putting al @-@ Musta 'sim in a difficult position . 
