Hittite's vassal treaty system introduced by Suppiluliuma I

To make the background a little bit clearer for better understanding, it would be useful to go back the time to Suppiluliuma I’s reign, where the seed of Hittite’s issues was sown:

‘Without any known exceptions, previous usurpers came to the throne through assassination, not civil war. The reason for this was fairly straight forward. Until the empire (introduces the vassal treaty system), armies were led by generals who were appointed by the king on a campaign-by-campaign basis. Therefore a general had little opportunity to build up a power base against his sovereign.

The vassal treaty system would ultimately undermine this system. Šuppiluliuma I introduced the widespread use of the treaty to control vassal kings. His reasons were undoubtedly sound. He made treaties with kings of distant lands which he could not reasonably incorporate into the closely controlled provincial system. But, from the very beginning, this system demonstrated a dismal record for maintaining a vassal’s loyalty. Even worse… this system was internalized by Muwattalli(sh) II when he created the kingdom of Hakpiš for Hattušili. This may have meant a reduction in imperial expenditure on this deeply troubled region, but it also meant that there was now an army whose loyalty was centred around the vassal king(Hattusili), rather than upon the Great King (Muwattallish II). Whether or not he realized it, Muršili III (legitimate successor of Muwattalish II) undoubtedly had the right idea when he tried to eliminate this threat to his authority. In the end, however, Muršili proved unable to undo the damage done by his father. Hattušili used the army of Hakpiš to defeat the imperial army and seize the imperial throne (http://www.hittites.info/history.aspx?text=history%2FLate+Late+Empire.htm).

Even though Hattusili III, attempting to establish his legitimacy and authority, forced ‘the loyalty oath which the Men of Hatti were required to swear to the new Great King’ (ibid), it seems that, due to the vassal treaty system, his reign failed to bring back stability to the empire. Especially in the west part, it is said that ‘Hattušili rapidly lost control of the situation’ (ibid) because vassal kings ‘were sworn to support the legitimate king, and to attack a usurper. If ever a vassal wished to throw off the yoke of Hittite rule, he was now presented with the perfect excuse to do so’ (ibid).

Suppiluliuma I

Further reading: http://wrex2009.wordpress.com/2013/02/03/the-battle-and-the-treaty-of-kadesh/