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[en] Of the 30 or so Galactic magnetars, about 8 are in supernova remnants (SNRs). One of the most extreme magnetars, 1E 1841−045, is at the center of the SNR Kes 73 (G27.4+0.0), whose age is uncertain. We measure its expansion using three Chandra observations over 15 years, obtaining a mean rate of yr−1. For a distance of 8.5 kpc, we obtain a shell velocity of 1100 km s−1 and infer a blast wave speed of 1400 km s−1. For Sedov expansion into a uniform medium, this gives an age of 1800 years. Derived emission measures imply an ambient density of about 2 cm−3 and an upper limit on the swept-up mass of about , with lower limits of tens of , confirming that Kes 73 is in an advanced evolutionary stage. Our spectral analysis shows no evidence for enhanced abundances as would be expected from a massive progenitor. Our derived total energy is erg, giving a very conservative lower limit to the magnetar’s initial period of about 3 ms, unless its energy was lost by non-electromagnetic means. We see no evidence of a wind-blown bubble as would be produced by a massive progenitor, or any evidence that the progenitor of Kes 73/1E 1841−045 was anything but a normal red supergiant producing a Type IIP supernova, though a short-lived stripped-envelope progenitor cannot be absolutely excluded. Kes 73's magnetar thus joins SGR 1900+14 as magnetars resulting from relatively low-mass progenitors.