Išme-Dagān I was the fortieth king of Aššur according to the Assyrian King List [http://oracc.museum.upenn.edu/riao/kinglists/assyriankinglist/index.html#Section5-7] (AKL). The AKL states that he succeeded his father, Samsī-Addu I [http://oracc.museum.upenn.edu/riao/fromsamsiaddutomittanicilent18081364bc/samsiaddudynasty/samsiaddui/index.html], to the throne and that he himself ruled for either forty or fifty years.
Although no royal inscriptions of this monarch survive, he is known from other texts dating both to his own reign and to that of his father. Samsī-Addu [http://oracc.museum.upenn.edu/riao/fromsamsiaddutomittanicilent18081364bc/samsiaddudynasty/samsiaddui/index.html] installed, during the latter portion of his reign, Išme-Dagān and another of his sons, Yasmah-Addu, on the throne at Ekallātum and Mari respectively. Thus Išme-Dagān had jurisdiction over the eastern portion of Samsī-Addu's [http://oracc.museum.upenn.edu/riao/fromsamsiaddutomittanicilent18081364bc/samsiaddudynasty/samsiaddui/index.html] kingdom while his younger brother was responsible for the west. Samsī-Addu [http://oracc.museum.upenn.edu/riao/fromsamsiaddutomittanicilent18081364bc/samsiaddudynasty/samsiaddui/index.html] himself resided in the centrally located Šubat-Enlil. From here he corresponded with his sons concerning matters of state (Ziegler 2006-08, 633). Many of these letters survive, along with letters between the two brothers (Heimpel 2003, 544-545).
Both Išme-Dagān and his father resided in Aššur periodically; however, it was not the main residence of either monarch. Išme-Dagān's primary wife was named Lamassī-Aššur and may have been an Assyrian princess (Birot et al. 1979, 143; see also Marello 1993, 271-279). There is evidence that Yasmah-Addu travelled to Aššur several times and took part in religious ceremonies along with his father, brother, and sister-in-law. Therefore, D. Charpin (2004, 379) suggests that the kingdom of Ekallātum had two capitals: Ekallātum as political capital and Aššur the religious one.
The AKL's assertion that Išme-Dagān's reign lasted for 40/50 years is disputed by modern scholarship. Landsberger (1954, 36-37), Veenhof (1985, 212), and Gasche et al. (1998, 52) all argue that this figure is likely too large. Landsberger and Veenhof both suggest that this number takes into account the years in which Išme-Dagān acted as viceroy in Ekallātum (before his father's death). Landsberger states that this reduces Išme-Dagān's reign to twenty years. Gasche et al. go yet further and reduce Išme-Dagān's reign to a mere 11 years. It is certainly the case that Išme-Dagān's reign was somewhat unstable (Charpin 2004, 380).
Several letters from Mari indicate that Išme-Dagān I was forced to leave the throne in Ekallātum after the invasion of the Elamites and seek exile in Hammurapi's court in Babylon (Gasche et al. 1998, 52; Charpin et al. 1988, 154-156). Išme-Dagān is not attested after Hammurapi's thirtieth regnal year and presumably died around this time (Charpin et al. 1988, 155-156). Although he seems to have been succeeded by his son, Mut-Aškur [http://oracc.museum.upenn.edu/riao/fromsamsiaddutomittanicilent18081364bc/samsiaddudynasty/mutashkur/index.html], the transition between the kings was not smooth (see introduction [http://oracc.museum.upenn.edu/riao/fromsamsiaddutomittanicilent18081364bc/samsiaddudynasty/mutashkur/index.html] to Mut-Aškur's reign).
It is possible that later tradition remembered Išme-Dagān I as a great monarch. Landsberger suggests that sometime after the reign of Tiglath-Pileser I [http://oracc.museum.upenn.edu/riao/thekingdomofassyria1114884bc/tiglathpileseri/index.html] (1114-1076 BC), Išme-Dagān I's popularity in historical memory rose to the degree that his immediate successors (Mut-Aškur [http://oracc.museum.upenn.edu/riao/fromsamsiaddutomittanicilent18081364bc/samsiaddudynasty/mutashkur/index.html], Rīmu-x [http://oracc.museum.upenn.edu/riao/fromsamsiaddutomittanicilent18081364bc/samsiaddudynasty/rimux/index.html], and Asīnum [http://oracc.museum.upenn.edu/riao/fromsamsiaddutomittanicilent18081364bc/samsiaddudynasty/asinum/index.html]) were not included in the main version of the AKL.
Poppy Tushingham
Poppy Tushingham, 'Išme-Dagān I', The Royal Inscriptions of Assyria online (RIAo) Project, The RIAo Project, a sub-project of MOCCI, 2021 [http://oracc.museum.upenn.edu/riao/fromsamsiaddutomittanicilent18081364bc/samsiaddudynasty/ishmedagani/]