EMB-145 FLT CREW ON VIS APCH RECEIVES A DESCENDING TCAS RA FROM AN F15 ON AN OVERHEAD APCH.

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WE WERE CLRED FOR A VISUAL APCH INTO ZZZ RWY XXR PERHAPS 10 MI OUT. NEAR THE FAF; WE WERE TOLD THAT AN F15 WOULD BE FLYING OVERHEAD FOR AN OVERHEAD BREAK FOR OUR RWY (THIS IS RATHER COMMON IN ZZZ). WE BEGAN OUR DSCNT ON THE GLIDE PATH AND NOT SOON AFTER WERE GIVEN A TFC WARNING BY TCAS SOON AFTER FOLLOWED BY AN RA TELLING US TO DSND. THE FO TOOK IMMEDIATE ACTION TO FOLLOW THE DSCNT RATE GIVEN AND I NOTED THE TCAS TARGET WAS A MERE 400 FT ABOVE US. THIS; OF COURSE; PUT US IN A DIFFICULT POS AS WE HAD NO INTENTION OF FLYING THE ACFT INTO THE GND. THE DSCNT RATE INDICATED ON THE VSI WOULD HAVE PUT US DANGEROUSLY CLOSE TO THE GND. I NOTED THAT THE F15 HAD STOPPED DSCNT AND HAD PASSED US; SO I COMMANDED THE PF TO EASE THE DSCNT. SIMULTANEOUSLY; I WAS ALSO ALERTING THE TWR OF THE TCAS COMMAND AND OUR CORRECTION. WE WERE 2 DOTS BELOW THE SLOPE AT APPROX 1200 FT AGL. THE TWR OFFERED RWY XXL FOR LNDG AND WE ACCEPTED; GIVEN THAT WE WERE IN A PERFECT POS FOR THAT RWY AND WITHIN THE REQUIREMENTS OF THE SOP. THE FLT CONTINUED NORMALLY. IN ADDITION; HAD WE NOT BEEN SO FAMILIAR WITH THE ARPT AND APCH; IT WOULD HAVE BEEN MUCH MORE CHAOTIC. I BELIEVE THAT ATC FAILED TO PROVIDE SEPARATION OR THAT THE F15 PLT FAILED TO MAINTAIN PROPER SEPARATION. WE AS THE CREW MUST REACT TO AN RA WHEN WE DO NOT HAVE THE TARGET IN SIGHT. HOWEVER; DSNDING INTO TERRAIN IS ALSO NOT AN OPTION. AT THE TIME; THE INCIDENT SEEMED QUITE BENIGN; BUT AFTER FURTHER THOUGHT I REALIZED THE GRAVITY OF THE INCIDENT. IF HE HAD CONTINUED HIS DSCNT WE WOULD HAVE BEEN FORCED TO TAKE AN UNKNOWN RISK IN TRYING TO ESCAPE BEING DSNDED UPON OR FLYING INTO THE TERRAIN. THIS IS AN EXTREMELY UNUSUAL SITUATION THAT I BELIEVE SHOULD HAVE NEVER HAPPENED IN THE FIRST PLACE IF PROPER SEPARATION PROCS WERE USED. ALSO ADDING TO FURTHER COMPLEX THE SITUATION WAS THE F15'S USE OF UHF RADIO CHANNELS THAT WE ARE UNABLE TO HEAR. WE NEVER HEARD HIS REACTION OR INTENTIONS DURING THE INCIDENT. THIS IS A SCENARIO THAT WOULD BE DIFFICULT TO TRAIN FOR AND RESULTED IN OUR ACTION OF 'SEAT OF THE PANTS FLYING.' IT WOULD BE ADVANTAGEOUS TO KNOW WHAT THE RECOMMENDED PROC IS IN THIS TYPE OF SITUATION SHOULD IT HAVE EVOLVED FURTHER.