The “scallop war” that erupted Tuesday morning between French and British fishermen in the English Channel is an unintended byproduct of the Brexit crisis. Coming amid growing concern that London and the European Union (EU) may fail to reach a friendly Brexit settlement, it is a warning of the many unexpected conflicts that could erupt, amid the relentless stoking of nationalism by the ruling elites on both sides of the Channel.

On Tuesday, 40 smaller French boats and five British fishing vessels clashed violently, throwing rocks and smoke bombs and ramming each other. Dimitri Rogoff, the president of the Normandy regional fishermen’s committee, recounted the naval clash on Tuesday to Le Figaro: “Around 40 ships put to sea at night to denounce British fishermen who are pillaging the scallop fields. The French went to clash with the British and prevent them from working. There was contact.”

The British ships withdrew from the area in the Seine Bay as they risked being surrounded, and the French coast guard allegedly refused to intervene.

Barrie Deas, chief executive of Britain’s National Federation of Fishermen’s Organizations, said, “We are advising all parties to be calm, as from the video clips, some vessels are manoeuvring very dangerously. … The deeper issues behind the clashes should be settled by talking around the table, not on the high seas where people could be hurt.”

Deas echoed calls from British fishermen for the Royal Navy to be dispatched to French waters, in an echo of the 1958–1976 “cod wars” between Britain and Iceland, during which the two countries’ navies rammed each others’ vessels to try to seize cod fishing grounds around Iceland. Deas told the BBC: “This is well beyond legal behavior. We have asked the British Government to intervene at a diplomatic level but also to provide protection for our vessels.”

For now, the British and French governments are trying to downplay the incident. Asked about the matter, British Prime Minister Theresa May called for a negotiated settlement: “I think it’s important we see an amicable solution to what has happened in the Channel. It’s what we want and it’s what France wants, and we will be working on that.”

George Eustice and Stéphane Travert, the British and French ministers of agriculture, have discussed the clashes. In France, Europe1 radio noted, “Stéphane Travert has not yet spoken on this sensitive issue—unsurprisingly, as European regulations put France in the wrong in this case.”

Nonetheless, the clash highlights how the prospect of Brexit is dangerously inflaming longstanding economic tensions between the European powers. Fifteen years ago, to prevent over-fishing, France unilaterally limited the scallop fishing season for French fishermen to October 1–May 15. Under EU rules, however, British and Irish boats were not subject to French national regulations. Therefore, in French waters outside the territorial exclusion zone within 12 nautical miles of the French coast, British and Irish ships continued legally harvesting scallops year-round.

After similar clashes over scallops in 2012, agreements had been signed to limit conflicts between the fishermen, as the French objected to the British ships’ fishing in summer and accused some British ships of also illegally fishing in French territorial waters. Talks on this year’s agreements broke down, however, as Britain’s exit from the European Union (EU) looms next March.

While British Prime Minister Theresa May is calling for a negotiated “soft Brexit” maintaining substantial ties with the EU, a “hard Brexit” or a “no-deal Brexit” with no commercial agreements reached between London and the EU would see Britain exit EU fisheries agreements, as well.

Statements from the scallop fishermen make clear the issues behind the clash are still unresolved.

From Normandy, Rogoff bitterly complained, “For the British, it’s open bar: they fish when they want, where they want and as much as they want. We don’t want to prevent them from fishing. But they should at least wait until October 1 so we can all share it together!” He added that fishermen in Normandy expect Brexit to keep British vessels from fishing in the area: “Normally, after 29 March 2019, they will be considered an external power and will no longer have access to these zones.”

French fisherman working in small, 15-meter boats also complain about the ecological impact of British boats twice their length, that dredge large numbers of scallops and undermine attempts to manage the scallop population. One British fisherman told the Guardian off the record: “Dredging is an awful kind of fishing. Not all fishermen want to be dredgers. It leaves the marine environment in a terrible mess.”

Whether or not London and Paris cobble together a settlement of the scallop dispute, the incident points to the bitter, nationalist mood in the media and political establishment in the run-up to Brexit next March, and the potential for even more violent conflict. The British Daily Telegraph forecast, “The scallops row is just the beginning: Brexit will trigger a full-blown fish war with the EU.”

With EU-British relations and hundreds of billions of euros in EU-British trade at stake, financial markets and ruling circles are on edge. After French Prime Minister Edouard Philippe declared on Monday that Paris is preparing for a “no-deal Brexit,” EU Brexit negotiator Michel Barnier’s apparently more favorable statement sent the pound up 1.2 percent on Wednesday. Barnier promised London a post-Brexit “partnership with Britain such as has never been with any third country” and indicated that the EU might give Britain more time to negotiate a deal.

The pound fell subsequently as Barnier walked back his statement, meeting with German Foreign Minister Heiko Maas and ruling out “à la carte” UK access to EU markets.

While French media reported British Twitter postings calling for the Royal Navy to open fire on French fishing boats, Sébastien Jumel, the Stalinist mayor of the French coastal city of Dieppe, wrote a letter to Travert backing the French fishermen. He wrote, “Our French fishermen set up traditional fishing methods that respect natural resources. But they increasingly face British-flagged fishing vessels, some over 30 meters long, that carry out massive and irresponsible industrial fishing, dangerously eroding sea resources.”

With French fishermen getting up to 40 percent of their catch in British waters, however, a no-deal Brexit would have broad, unforeseen economic repercussions in France, as well. Last year, Travert warned a fisherman’s conference in Sète: “If on 30 March 2019, by some misfortune, the United Kingdom decides to suddenly cut its ties to the European Union, without a negotiated withdrawal and therefore a transition period, the consequences will be brutal and immediate.”