Yes, but generally(not necessarily always), a nation/kingdom will initiate a war only when they truly feel they can win.
(Romans, Persians to Greeks, Germans in WWI and II, the list goes on) Now that doesn't necessarily mean that victory is assured per say, but it does mean that the aggressor nation feels fairly confident that when they invade x nation, they will have a high probability of coming out on top.
However there are plenty of ways to roleplay the declared war which make more sense than some sort of affair. The war could have been a desperation move. The French in this circumstance are blocked in by British Navies and have nowhere to go. To their west they see a nice chunk of land which they may be able to pull off an attack with such speed so as to win quickly and add new ports with new sources of income and a larger population to field armies from.
Does that sound sensible to you? It does to me, for the Athenians did essentially the exact same thing in the Peloponnesian War when they attacked Syracusae.
As to them not attacking on the re-roll, on that occasion, they may have assessed their power and war making capabilities, and decided that a war with you would have been a lost cause.
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