I came across this little factoid while reading a book on the 18th century. For a bit of background: at this point the generally accepted approach to the laws of war at sea was that neutrals were still allowed to trade with whoever they wanted and should not be harrassed by either of the two sides in a war while trading with both. However, because Britain had an advantage in sea power from the mid-18th century on, we started trying to impose an alternative interpretation of the law which said that neutrals could carry on with any trade they had been conducting before war broke out, but if they started transporting additional stuff from a power hostile to Britain after war broke out, their ships were fair game to be stopped and confiscated. This had already led to tensions during the Seven Years' War when the Dutch took over the sugar trade from the French colonies to France and the Royal Navy kept stopping their ships.
Now, the best known impact of this during OTL's American Revolutionary War was that the Dutch continued to trade with the rebel American colonies and Britain's opposition to this led to the Fourth Anglo-Dutch War as an additional 'front' of the ARW. However, I found out that there is a flip side to this. In 1780 the Spanish stopped two Russian ships that were running food supplies to the besieged British garrison in Gibraltar, using the same interpretation of the law. Catherine the Great was furious at the humiliation of her subjects and wanted to declare war on Spain. She was eventually talked out of it by one of her ministers and instead just issued a declaration on behalf of the Armed Neutrality of the North condemning all attacks on neutral shipping.
What if Catherine hadn't been talked out of it, and Russia declared war on Spain in 1780?
The effect on the outcome of the ARW is probably minimal, but I think it would have interestingly significant effects on how the war is viewed. OTL, Tsarist Russia was the USA's most reliable (and ironic) ally in the 19th century, which could conceivably be prevented by this. Also, many British officers refused to fight the Americans in the early part of the ARW because they believed the American cause was just, but joined the war after the Americans welcomed the despotic French and Spanish as allies, now seeing the rebels as hypocrites. The presence of despotic Russia on the British side might give them pause, and lend fuel to American propaganda efforts.
Finally I could see this having an effect on the Nootka Sound dispute. Russia might try to stake a claim to California as part of the war and Britain might drop her own claims in order to encourage the Russians to join the war.
Thoughts?