Romulus being close to the border doesn't preclude there being an enormous Empire behind them. It seems perverse to suggest the Romulus were one planet, surrounded by Federation bases, given the way they've always been depicted as serious threats to the Federation and the Klingons. The intention always seems to have been to have them as equals.
Far from Cuba in your analogy, they are China. It's possible Nanclus' presence in TUC means there was a period of détente between the Feds and the Romulus. Like the Soviets and the Chinese, the once-allied Klingons and Romulus now hate each other more than either distrust the Federation. So we get something like Kissinger's triangular diplomacy, the fruits of which are a short-lived Planet of Galactic Peace on Nimbus III.