the as sombreru pillbox i, as sombreru pillbox ii, and as sombreru pillbox iii are three historic defensive fortifications on guam, near tumon, that were listed on the u.s. national register of historic places in 1991. they are pillboxes, a type of defensive fortification, built by the 48th independent mixed brigade; 29th division of the imperial japanese army during 1941-1944, between japan capturing guam on december 8, 1941 from the united states and the u.s. recapturing guam in 1944.