Brothers in arms - An experiment on the alliance puzzle

► We study contests (between a two-player alliance and a single player) experimentally. ► We compare effort choices in inter-alliance contests under peaceful and non-peaceful sharing rules (within the victorious alliances). ► We show that in-group solidarity vanishes after the break-up of the alliance. ► Former 'brothers in arms' fight even more vigorously (afterwards) against each other than strangers do. ► This vigorous internal fighting is anticipated and reduces alliancesʼ ability to mobilize their joint fighting effort.