  We present results of a 400-ks Chandra observation of the young shell
supernova remnant (SNR) G11.2-0.3, containing a pulsar and pulsar-wind nebula
(PWN). We measure a mean expansion rate for the shell since 2000 of
0.0277+/-0.0018% per yr, implying an age between 1400 and 2400 yr, and making
G11.2-0.3 one of the youngest core-collapse SNRs in the Galaxy. However, we
find very high absorption ($A_V \sim 16^m \pm 2^m$), confirming near-IR
determinations and ruling out a claimed association with the possible
historical SN of 386 CE. The PWN shows strong jets and a faint torus within a
larger, more diffuse region of radio emission and nonthermal X-rays. Central
soft thermal X-ray emission is anticorrelated with the PWN; that, and more
detailed morphological evidence, indicates that the reverse shock has already
reheated all ejecta and compressed the PWN. The pulsar characteristic
energy-loss timescale is well in excess of the remnant age, and we suggest that
the bright jets have been produced since the recompression. The relatively
pronounced shell and diffuse hard X-ray emission in the interior, enhanced at
the inner edge of the shell, indicate that the immediate circumstellar medium
into which G11.2-0.3 is expanding was quite anisotropic. We propose a possible
origin for G11.2-0.3 in a stripped-envelope progenitor that had lost almost all
its envelope mass, in an anisotropic wind or due to binary interaction, leaving
a compact core whose fast winds swept previously lost mass into a dense
irregular shell, and which exploded as a Type cIIb or Ibc supernova.
