  The pressure- and anion-dependent electronic structure of EuX (X=Te, Se, S,
O) monochalcogenides is probed with element- and orbital-specific X-ray
absorption spectroscopy in a diamond anvil cell. An isotropic lattice
contraction enhances the ferromagnetic ordering temperature by inducing mixing
of Eu 4{\it f} and 5{\it d} electronic orbitals. Anion substitution (Te $\to$
O) enhances competing exchange pathways through spin-polarized anion {\it p}
states, counteracting the effect of the concomitant lattice contraction. The
results have strong implications for efforts aimed at enhancing FM exchange
interactions in thin films through interfacial strain or chemical
substitutions.
