Fortress Calais

If Mary doesn't lose Calais by not following her husband into war with France, could England keep hold of Calais up until the present day or what's the latest plausible date Calais would fall to the French.

England kept the French pretty much at bay throughout the reign of Elizabeth I so it could remain English until 1603. The Duke of Buckingham pushed James I for a war with France so it could conceivably be lost by this point. However, Elizabeth could fortify the city during her reign, making it impossible or near impossible to capture. Re-supply was only 24 hours away so it could be done. Possibly, maybe.

England and subsequently Britain was able to defeat the French in various wars (With the exception of the American war of Independence) after the reign of King Charles II so Calais were overrun, it could be returned via peace treaty. The tricky part is keeping Calais English from 1603-1680. Can it conceivably be done? What impact would this have on history? Would the Channel Tunnel be build 100 years sooner? With its strategic importance, would Calais swell in size to become a major European trading centre, population in the millions?

Interesting- so Calais becomes a French Hong Kong as it were...

Would it stop with Calais? The holding of it would necessitate a larger British Army(English technically) making England more formidable in the world. Also would require certain naval forces for protection and supply. Then, given the ties for some time between the English and Dutch royalty, and the English(later British) interest in what would become Belgium...

Well, Britain still possesses a European version of Hong Kong. It's called Gibraltar. The only reason the British were able to hold onto it was that nobody was able to take it in the American Revolution or the Napoleonic Wars. After that, there really would never have been a serious threat to Gibraltar unless Franco joined the Axis, but even then, it would still have been difficult for Gibraltar to fall.

Calais is an entirely different story. France would be fully capable of taking the city in the 18th and 19th Centuries. Perhaps one condition of the Seven Year's War would be Calais in exchange for Canada? Or even if the French don't manage to retake Calais in the Seven Year's War, it would create interesting leverage for the French to go on a war of revenge to retake Calais under the guise of supporting the American cause.

But I'm afraid having a French city in British hands will cause a tremendous amount of butterflies in the long run.

Calais was bad, it was too hard to defend just being one city and was a major budget drain. Without Calais England could fully concentrate on its navy and getting around. Keeping Calais would restrict English development.

With Calais in British hands, Britain could have more troops on the continent in time for WW1. An even faster British deployment with more troops could really screw up the advances of the Germans.

WW1?
LOL, thats pushing it...

I would think that Britain would lose Calais either by the Seven Years Year, the American War of Independence or the Napoleonic Wars. After any one of them I could see the British Government considering the loss a blessing in disguise.

I would think even earlier, Vauban would try his hand on...

There would be no American rebellion or 7 years war, the butterflys from this would be rather big.
England's emphasis would remain focussed on regaining its land in France.