In 2022, the “Will to Fight Act” was referred to the US Congress urging attention to measuring and assessing will to fight. That Bill was not enacted, and evaluation efforts within the political and military establishment remain contentious, fragmented, and meager. This likely will persist, along with attendant policy failures and grievous costs, without awareness of research that the social and psychological sciences reveal on the will to fight [S. Atran,
Science
373
, 1063 (2021)]. We illustrate such research using converging data from a multimethod and multicultural approach, including field and online studies from the Middle East, North Africa, and Europe. These studies reveal specific psychosocial pathways, within a general causal framework, that predict willingness to make costly sacrifices, including to cooperate, fight, and die in war and sustained conflict. From the continuing strife in Iraq to embattled Ukraine, 31 studies were conducted in 9 countries with nearly 12,000 participants. These include people in longstanding conflicts, refugees, imprisoned jihadists and gangs, US military, studies in Ukraine before and during the current war, and rolling studies with a European ally of Ukraine. Results provide evidence for a mediation model of transcultural pathways to the will to fight. Building on our previous behavioral and brain research, on the battlefield in Iraq, with violent extremists, and with US military, the linear mediation yielding the will to fight involves identity fusion, perceived spiritual formidability, and trust. The model, a variation on “The Devoted Actor Framework,” applies to primary reference groups, core cultural values, and leaders.