ACR MADE A TURN ON COURSE RESULTING IN LTSS WITH A MIL ACFT.

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ACR X; RIC TO ORD AND MIL Y AN MLT LFI TO EVV. MIL Y WAS AT FL240 HDG 310 DEG ON A VECTOR TO MOL. (MOL VOR OTS). I TOOK A HDOF ON ACR X CLBING TO FL200. ACR X WAS TRACKING IN A WESTERLY DIRECTION. I POINTED ACR X OUT TO APPROPRIATE SECTOR AND GOT PERMISSION TO CLB. CLBED ACR X TO FL230 AND PUT HALO ON TARGET SO I COULD CLB WHEN CLR OF MIL Y AT FL240. WHEN MIL Y WAS OBSERVED OUTSIDE OF THE HALO I CLBED THE ACR X TO FL270. ABOUT THE TIME THE ACFT STARTED CLBING IT TURNED TOWARDS MOL. THIS CAUSED THE ACFT TO START CONVERGING INSTEAD OF CONTINUING TO DIVERGE. I TURNED ACR X 'IMMEDIATELY' TO HDG 270. WHEN ACFT WERE CLR OF EACH OTHER AGAIN I HANDED ACR X OFF AND ISSUED RADAR VECTOR TO REJOIN FILED RTE. I HAD A VISITOR OBSERVING AT THE SECTOR AT THE TIME. I SHOULD HAVE MADE MYSELF MORE COGNIZANT OF THE FACT THAT ACR X WAS DUE TO MAKE A TURN AND NOT CONTINUE ON A WESTERLY HDG. THERE WERE ONLY 2 ACFT IN THE SECTOR AT THE TIME. I DIDN'T HAVE THE FLT PROGRESS STRIP ON ACR X YET. I SHOULD HAVE CHKED THE RTE OF INTENDED FLT PRIOR TO ISSUING CLB CLRNC.