The Age of War was not kind to the Tamar Pact, as many of its planets fell to the Draconis Combine. By 2445, a full fifth of the Pact was under Kuritan rule.  Later during the reign of Archon Margaret Olsen, the machinations of the Duke of Tamar resulted in a civil war which finally ended in battle on Tamar in 2405, when the new Archon Robert Steiner stripped the House Tamar of its titles and rank. It was here that the House Kelswa came into power, and they held the Dukedom of Tamar for the next six-and-a-half centuries.  Indeed, the Kelswas had long been an important and loyal family within the Pact, with Prime Minister Davis Kelswa in power as early as 2364. 
As the various Great Houses became increasingly overt in their raiding during the Hidden Wars towards the end of the Star League era, the Tamar Pact found itself facing the brunt of the "pirate raids" instigated against the Commonwealth. The Duchess of Tamar during the mid twenty-eight century, Monique Kelswa, had only a rudimentary knowledge of military strategy, and instead appointed military advisors to help her prepare the Tamar Pact against acts of aggression. Among her policies applied across the Pact were tax breaks for defense industries and the implementation of state-sponsored grants for higher education on rural worlds within the Pact. These measures led to a steady drop in unemployment and a boom in university enrollments and made the Duchess a promising future leader in the minds of many despite her becoming Duchess at a very young age.
The Clan Invasion was devastating to the Tamar Pact, which bore the brunt of the attacks by both Clan Jade Falcon and Clan Wolf. Tamar itself fell, with Duke Kelswa fighting to the bitter end. Selvin Kelswa's daughter, Morasha Kelswa inherited her father's now meaningless titles. 
By 3054, the Tamar March had been reduced to little more than a sliver of its former glory. Nevertheless, as it essentially included the entire Federated Commonwealth-Clan Occupation Zone border, it was the most heavily defended area of the Commonwealth. 
- House Steiner (The Lyran Commonwealth) (sourcebook) p. 20
- House Steiner (The Lyran Commonwealth) (sourcebook) pp. 27-29
- House Steiner (The Lyran Commonwealth) (sourcebook) p. 12
- Era Report: 2750, p. 81, "Monique Kelswa"
- Wolf Clan Sourcebook pp. 43-45, 64
- Objective Raids pp. 22-23
- Jihad Hot Spots: Terra, p. 19, "Who Is Gregory Kelswa-Steiner?"