Ethiopia and Eritrea are locked in what I call a 'mutuality of strategic regret' - each side rueing missed opportunity to 'finish the job', alter political geography and create permanent deterrence. Eritrea fell out with Ethiopia after Abiy accepted Pretoria peace accord in 2022 and allowed TPLF to reconstitute and govern again when - from Asmara's perspective - the strategic end goal was the total capitulation and complete evisceration of the group. Asmara hoped entrenched strategic influence in Addis and a permanently neutered Tigray would allow it regain contested borderlands, guarantee its long-term survival and confer it with strategic depth. Eritrea regrets not to having 'finished the job' after Pretoria when it had the troop strength, presence and opportunity, in addition to 'regret' at not being able to apply maximum pressure on Abiy to 'stay the course.' The TPLF in the final phases of the Tigray War working in coordination with OLA pushed hard towards capital Addis from multiple axes, at extraordinary cost, then stopped abruptly, primarily in response to direct American pressure and warning not proceed. That decision to succumb to external pressure, not to 'finish the job' and take the city, overthrow the Abiy government has since continued to rankle with TPLF hardliners. PM Abiy Ahmed, in the last four years, has consistently framed the independence of Eritrea and the decision to cede the Red Sea coast as a 'catastrophe' for the Ethiopian nation and state; a strategic blunder that 'must be corrected.' A lethal mutuality of regret binds the belligerents, animates the war calculus of the two adversaries. Familiarity with this dynamic is essential to understand the new momentum toward war...