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From "Thomas Cochrane's Fighting Ships, 30th Edition, copyright 1970,
Cochrane Naval Press, Annapolis, CNA"

"...In mid 1943, the NUSM Puerto Hancock design bureau was tasked with
the development of a company-strength submersible troop transport.  It
was already becoming apparent as early as 1942 to operational-level
echelons in Siberia that maintaining air-superiority over broad swaths
of the continental front-lines, the littoral and Japan itself was
becoming an ever more difficult task, making the small scale
amphibious strikes in use as an operational flanking technique a dicey
proposition at best.  Still, amphibious operations were seen as a
strategic sideshow and it seems likely that the concerns of local
commanders would likely never have advanced as far as Chapultepec
Castle had it not been for serendipity.  At Silva's prompting,
concepts for a "knockout blow" against Japan were being floated during
this period, and planners seized on the idea of submarine transport as
a hedge against the increasingly strong Japanese presence in the air
over the Home Islands.

The original 1942 design concept of a mid-range diesel-electric of
about 3,000 tons displacement (already a fairly large boat) capable of
delivering a company of marines was transformed into the
_Sirena_-class, 6,500 ton surface-displacement monstrosities capable
of delivering a battalion of heavy infantry along with motorized
transport or, as the _Conquistador_ sub-class, an armored company of
12 terramobiles.  Range was cut to a bare minimum as the boats were
optimized for the Siberia-Japan crossing.  The troops and vehicles
would be unloaded over a bow ramp, coming under the support of 4
bow-mounted .50 caliber autocannons.

9 _Sirenas_ and 3 _Conquistadors_ were completed in time for the
invasion of Honshu in 1944, and all participated.  _Hippocampo_ was
lost on approach to Honshu with all hands due to a magnetic mine, but
the 11 other craft delivered their charges, achieving complete
operational surprise.

The _Sirenas_ continued in use in-theater for the duration of the
Mexican presence, used first to ferry troops and supplies to Japan
under the increasingly effective Imperial Japanese Air Arm and then
later during the evacuation of Mexican and Siberian troops from the
collapsing position.  It was during the final days of the pocket's
existence that the _Sirena_ herself was lost, coming under dive-bomber
attack as she was on-loading elements of the 132nd Infantry Regiment
for the return trip to Siberia.  This effectively signaled the end of
the organized evacuation as it became impossible for the NUSM to
approach the Japanese coast in any meaningful way.

The remaining _Sirenas_ were retrofitted for longer range operations
and spent the rest of the war in use, dispersed across the Pacific,
evacuating marines here and delivering a final load of supplies there.
 Post-war, the entire class were deemed surplus to the operational
needs of the NUSM and were transferred to the AFANG en masse in 1960
with the exception of _Cochise_ and _Grifo_, which were refitted as
technology test-beds."