Even EUIII had fairly extensive diplomatic options, including the requested trust building over time and mutual actions, as well as many ways to build or tear down Relations. Unfortunately, those mattered very little when it came time for the AI to decide what to do, because the Random Number Generator overrode just about all other concerns.
For example: Two big countries are at war, and one is getting the snot beat out of it, with its armies basically wiped out, half of its provinces already occupied, and the rest under siege. The losing country asks a small country for an alliance, despite TERRIBLE Relations, nothing to gain from the deal, and the fact that they were at war with each other only a year or two before. The small country gladly accepts (because the RNG says so) and joins the war, getting itself pounded flat in a matter of weeks. Duh. The bad relations didn't matter, the low trust didn't matter, the surrender progress of the war didn't matter, but the RNG certainly did. There's a difference between "deep" diplomacy and "credible" diplomacy.