They probably opened fire because there was nowhere to hide.
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The Manila Times
14 August 1924
More details have emerged on yesterday's sinking of the
R-10...
The ship had noted some unusual activity on a small atoll, and moved in to investigate. While preparing to launch a small boat with a landing party, a medium-caliber gun, concealed in a pile of brush, opened fire on them at very close range.
The ship took three hits, one below the waterline in the boiler room. Returning fire, the crew believes they scored a direct hit on the installation.
Taking on water and with half the engines unoperational, the ship attempted to return to Manila, but bad weather forced the crew to abandon ship halfway home. The
R-7, the other boat assigned to South China Sea patrolling, picked up the survivors.
While official sources have been silent, unofficial reports are that an operation is being planned to neutralise the pirate threat once and for all...