Who said, getting a faction to agree to become a protectorate is difficult? I suspect, the difficulty is tied to the personality trait assigned to each faction at the beginning of the campaign. I got Pontus to agree to become a protectorate after I had taken one province from them: I just turned the province back to them and threw in 10K to sweeten the deal... To my surprise, I discovered that Pontus had actually annihilated Armenia and Parthia and conquered several Seleucid provinces at that point... My province count went up from about 30 to 45 in one turn...
Meanwhile, Carthage would give me the freedom-value speech even if left with a single province, their only town being beseiged by my armies and the only port being blockaded by my boats...
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