This paper examines which structural forces account for U.S. business cycle fluctuations. It combines a trend-cycle Bayesian VAR with a joint max-share identification scheme that offers an alternative to set-identification under sign restrictions. Separating trend from cycle, the framework decomposes business cycle dynamics into four structural components-monetary policy, non-policy demand, cost-push, and oil supply shocks-consistent with New Keynesian theory. The results indicate that demand forces dominate real activity and account for a large share of nominal fluctuations at short horizons, while supply forces become pivotal for inflation at medium-run business cycle frequencies (6-32 quarters). Together, the four shocks account for most of the cyclical variation in both output and inflation.