  MSH 15-56 (G326.3-1.8) is a composite supernova remnant (SNR) that consists
of an SNR shell and a displaced pulsar wind nebula (PWN) in the radio. We
present XMM-Newton and Chandra X-ray observations of the remnant that reveal a
compact source at the tip of the radio PWN and complex structures that provide
evidence for mixing of the supernova (SN) ejecta with PWN material following a
reverse shock interaction. The X-ray spectra are well fitted by a non-thermal
power-law model whose photon index steepens with distance from the presumed
pulsar, and a thermal component with an average temperature of 0.55 keV. The
enhanced abundances of silicon and sulfur in some regions, and the similar
temperature and ionization timescale, suggest that much of the X-ray emission
can be attributed to SN ejecta that have either been heated by the reverse
shock or swept up by the PWN. We find one region with a lower temperature of
0.3 keV that appears to be in ionization equilibrium. Assuming the Sedov model,
we derive a number of SNR properties, including an age of 16,500 yr. Modeling
of the gamma-ray emission detected by Fermi shows that the emission may
originate from the reverse shock-crushed PWN.
