The allocation of scarce resources is a grand strategic question – burden-sharing behaviour has clear effects on states’ ability to contribute to collective defence. Both NATO and the European Union encourage members not just to spend more on defence, but to focus those expenditures on equipment modernization and shared operational requirements. After NATO allies formally pledged to improve burden-sharing along these lines in 2014, and EU members followed in 2016, transatlantic debates on defence spending have become increasingly tense, particularly since 2017. What actually drives states’ choices to allocate resources to shared defence priorities? I operationalize transatlantic burden-sharing in line with NATO’s ‘Cash, Capabilities, and Contributions’ approach for a mixed-methods analysis of the burden-sharing behaviour of NATO’s Black Sea littoral states – Bulgaria, Romania, and Turkey. Using a multi-method analytical approach, I find that national and particularly regional political economies drive burden-sharing choices more than geostrategy, at least in the current strategic environment.