Re: What do we know about the years between ST5 and ST6?
The really interesting thing is how Romulans now have ambassadors all over the place, including in secret top level UFP military meetings. Did the UFP go for the apparent "enemy of my enemy" alliance because the Klingons became worse than ever before - or because the Romulans played it nice?
Remember that the Romulans were considered the "good" bad guys of the TOS era. It's not too surprising that they'd be on better terms with the Federation than the Klingons, especially after the Genesis fiasco and the UFP's unwillingness to let the Klingons try Kirk for his alleged crimes against their government. How or why things changed after that (again with the Tomed Incident) is still a mystery as far as actual televised or filmed footage is concerned.
"He clapped his captain—his friend—on the shoulder. Yes, this man was very much like James Kirk, in all the ways that mattered." --Christopher L. Bennett-- Star Trek: Mere Anarachy, The Darkness Drops Again