On Sunday 16 August, war was declared between Germany and Britain. What? You think we got the date wrong? No, this is World War III, and it started in the small French inland port at Montereau, about an hour south of Paris by train. We were witnesses. We were there When. It. Happened. You heard it here first.

This time though, there was no assassination of an Archduke, and no Invasion of Poland. If anything, it could be argued that it was Britain that needed the lebensraum this time. Here's what happened...

Liz and I had left Pont Sur Yonne early, after a peaceful night moored beside a fun fair. That might sound like a contradiction in terms, but although the fair was indeed adjacent to the mooring - with dodgems, shooting galleries, food stalls, merry-go-rounds and every other attraction designed to lose you money - by about 11pm on Saturday night it was all over bar the shouting, and there wasn't any shouting. The good folk of Pont Sur Yonne were remarkable by their absence, having stayed away in droves. Sad for the fun fair folk though.

We cruised all morning and approached Montereau where the Yonne River meets the Seine, and eyed the mooring opportunities through the binoculars. Last time we were here a few weeks back, there was only one free space, into which we slotted neatly. This time there were two spaces, but both looked quite tight thanks to a small yacht which was poorly moored in the middle. Poorly moored? Yes...

Because there's a certain etiquette that goes with mooring boats against a quayside or pontoon. Basically, you leave as much room as you can for other boats when you tie up, which means butting up against any other boats already there, or moving all the way to the end so you don't hog the middle. However, at Montereau, a little yacht flying a German flag was moored inconsiderately halfway between two other boats, thereby using up valuable space. However, not to be put off we decided that the space in front of him was about 12 metres, and our boat is 11.4 metres, so we figured we could squeeze in.

We did, but not before the burly grey-haired German on Schweinhund II had expressed his consternation that we were getting uncomfortably close to his precious bow. By contrast, the Frenchman from the boat in front willingly came to help us moor and between him and Liz they tied Liberty up while I stayed at the controls to make sure we didn't back into Herman. I said to the Frenchman, 'All that man has to do is move his boat back a metre.' The Frenchman shrugged and said, 'Yes, but he's German I think'. I, of course, had no opinion.

Alles gut, c'est bien, she'll be right we thought, and tied off, retreating inside for a spot of lunch and to wile away the afternoon watching a DVD (too windy and cool to go exploring. We are wimps...).

Half way through the film we noticed the French boat in front had sneaked off, leaving the whole front of the pontoon free, so, to give Herman some breathing space we pulled Liberty forward as far as possible, leaving a good 13 metres between us.

If he sighed in relief it was short-lived, because literally within two minutes another boat arrived to squeeze into the space we'd just vacated - a low-slung older cruiser called Harmony, flying a red duster, so she was British. As the boat manoeuvred into place there was sudden Germanic screaming from Herman as he jumped up and down on his boat and turned bright red, pointing to his bow.

Meanwhile, on board Harmony, a shortish bearded Englishman muttered something about 'keeping your lid on' as Liz and I helped him moor. We got him tied up successfully, but Liz whispered to me that it looked like the rear of Harmony might possibly have mildly bumped Herman's bows; a glancing blow.

Herman leapt off his boat and shook his fist, turned even brighter red and seemingly declared war. The Brit meanwhile maintained a stiff upper lip and said to the German, 'Speak English or I'm not listening', then turned to me and said, 'Time for une tasse de thé (a cup of tea) and a fag.'

Apoplexy reached Guinness Book of Records' levels on the German side, and next thing Madame Capitanerie arrived asking what was happening. ''Qu'est-ce qui se passe?' Herman told her he'd been rammed, in a voice that sounded like he was addressing a Nuremberg rally. Unfortunately the Capitanerie woman didn't speak German, but we could all understand the word 'police', and Herman wanted them here, now, preferably armed and descending from helicopters.

Harmony's skipper denied scraping the Bismarck. Madame Capitainerie diplomatically told them both to sort it out through their 'assurance' - which is the French for insurance - and then Herman brought out the big guns - his digital camera. He took photos of the alleged scrape, Harmony, Harmony's skipper, us standing on the pontoon, and possibly a few sparrows on the bank, but he neglected to get any selfies showing how angry he was that he'd been invaded by Britain.

Mr Harmony meanwhile was sitting in his cockpit rolling a cigarette, and assured us all that he would provide Britain's insurance details to Germany in due course. ('I have in my hand a piece of paper'...)

As we know from history, most wars could have been avoided, and the same was true of this one. The German had something like 10 metres free space behind his boat, yet was reluctant to move back to make room for us when we arrived, and made no attempt at all when HMS Harmony rolled in. Instead he steadfastly clung to the space he had, and paid the price (the price being the tiniest of wavy lines on his bow paintwork, probably no longer than 10cms and which would likely come out with a bit of a rub with cutting compound. We had a tin on board, but were disinclined to offer it...).

Montereau is no stranger to battles; it was here in 1814 that Napoleon I successfully fought the Austrian Empire and the Kingdom of Würtemburg. Presently Montereau is planning a celebratory theme park, with the incredibly clever name of 'Napoleonland'. (Yes, really)

Depending on the outcome of the Battle of the Boats 200 years after Napoleon's skirmish, there might be room for another park: Britland. Or maybe Hermanland. Guess it depends who wins the insurance battle.