What weakness in the abbasid empire was revealed when tahir of khurasan

The  tahirid dynasty   (persian :   طاهریان ‎‎) was a dynasty, of  persian[3]  dihqan[4]   origin, that governed the  abbasid   province of  khorasan from 821 to 873 and the city of  baghdad   from 820 until 891. the dynasty was founded by  tahir ibn husayn , a leading general in the service of the  abbasid   caliph  al-ma'mun . their capital in khorasan was initially located at  merv   but was later moved to  nishapur. the tahirids enjoyed a high degree of autonomy in their governance of khorasan, but they remained subject to the abbasid caliphate and were not independent rulers. [3]

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