Does squad league exposure predict national team performance? Evidence from the 2026 FIFA World Cup

National teams increasingly rely on players competing in elite domestic and international leagues, yet it remains unclear whether this reflects a genuine performance advantage or simply the fact that stronger teams export more players to top-level competitions. Using open data from all 48 teams participating in the 2026 FIFA World Cup, we examined whether squad league exposure was independently associated with group-stage attacking performance after accounting for baseline team strength. Squad exposure was quantified using the proportion of players competing in Europe's top five leagues together with additional indicators describing squad composition. The primary outcome was group-stage expected-goals difference per match, while advancement to the knockout stage served as a secondary outcome. Associations were analysed using heteroscedasticity-consistent regression models, false-discovery-rate correction and repeated cross-validation. Teams with a greater proportion of players competing in Europe's top five leagues produced higher expected-goals differences during the group stage. However, this relationship was substantially attenuated after adjustment for pre-tournament Elo rating, indicating that most of the apparent association reflected underlying team quality rather than an independent effect of league exposure. Predictive analyses showed that squad composition explained only a modest proportion of performance variance and added little information once baseline team strength was known. Match-process variables remained considerably more informative for predicting tournament progression. Overall, squad league exposure is best understood as an indicator of established team strength rather than an independent determinant of attacking performance.