Not the quote I was actually trying to find (I clearly have too many books - oh who am I kidding!), but this from Front Line Kent, by Victor Smith, by Kent County Council
"After WWI the absence of an identifiable enemy initially produced uncertainty about priorities for home defence. After a period of disarmament, the need to achieve a balance of power with France evolved as a strategic requirement."
Within 'a few years' of 1920 "The only power then able to mount an attack was France".
"By the mid-1930's however defensive preparations were focusing on Germany".
Fear of French invasion of course wasn't new, and hadn't been completely vanquished along with Napoleon. From the same book "Fear of French invasion in the 1880's led to the contingency plan for large scale land defences of London". Some works were started then, and repurposed for 1939-45.
being so close to each other, and having plenty of historical conflicts to avenge, it would certainly be possibly to have seen France as the real enemy, especially with Germany being hit so hard after WWI. It could even be possible, at a stretch, to see a French-Spanish link up.
Let's give this a go, then.
The French political landscape starts to move towards radical socialism during and shortly after the war. The SFIO makes huge gains in the 1919 French legislative elections, and French newspapers tout the "Second French Revolution." The new socialist-dominated government intervenes in the Russian Civil War to aid the Bolsheviks, to the consternation of the Allies. The French objectives are: 1) to guarantee Liberte, Egalite, Fraternite to the Russians by backing the fledgling USSR, and 2) to ensure a French-aligned power exists in the East to threaten Germany.
However, French and Allied forces clash in Russia, and only the war-weariness of both sides keeps the conflict from escalating. Russia becomes a proxy war with occasional clashes between French and Allied forces. France and England are in a staring contest across the Channel, and even after the Allies withdraw from Russia, both sides keep their militaries ready for mobilization. France starts to build up her fleet, especially the brown-water navy, to defend her colonies. Meanwhile, the USSR builds up her fleet in the Pacific, hoping to fight the Russo-Japanese War 2.0. England sees the buildup as a threat to the island, and the cold war escalates as both sides sponsor independence movements in the others' empires.
France continues to keep Germany in a weakened state by occupying the Rhineland when Germany falls behind on payment of war reparations, as in OTL. England starts to improve relations with Germany, hoping to break Franco-Soviet dominance of mainland Europe. France responds to the perceived betrayal by pre-emptively invading Germany with their USSR ally. England sends the BEF to Germany's aid, but Germany falls quickly, and the war is continued in the colonies and on the high seas.
The Japanese, fearing Soviet dominance of the Far East, secretly allies with England. They plan to wait until the winter, when Vladivostok is iced over, then destroy the Soviet Pacific fleet and cut the Trans-Siberian Railway. However, the USSR sent the Baltic Fleet east to the Pacific as soon as war with England broke out, predicting correctly that the Japanese would join the war as allies of England. The Japanese plans to repeat their 1904 surprise attack on the Soviet fleet are spoiled when the Soviets surprise them instead, raiding the Japanese fleet with the Baltic fleet carriers and launching a successful offensive to retake Port Arthur.
Meanwhile, the French and British fleets have fought several skirmishes, with inconclusive results. Bombers fly back and forth across the skies to strike at economic targets, with mixed results. The British are keen to fight a decisive fleet action, and constantly harry the French ports with carrier strikes. The French, however, are less confident in their navy and use most of their fleet in peripheral attacks against British colonies in Africa and the Middle East.
The "Bore War", as the stalemate is known to the English, or "
Drôle de guerre" to the French, is broken when the Japanese fleet is destroyed by the Russians in an engagement that would be known as the last decisive action for the battleship, and the first for the aircraft carrier. Blockaded by Russian submarines and bombarded from the air, the Japanese refuse to surrender, but they will have no effect on the remainder of the war.
As the combined Baltic-Pacific fleets sail to Europe to aid their French allies, the Royal Navy is horrified by reports from RN observers in Japan. The RN knows it cannot allow a joint Franco-Russian fleet to exist in the Atlantic, lest England suffer the same fate as Japan. The RN steams south, planning to intercept the Russian fleet at the Cape of Good Hope. The RAF deploys every available squadron to South Africa to support the RN.
The Battle of the Cape of Good Hope, however, is a disaster for England. The RN is able to turn back the Soviet fleet, but at heavy losses. The RN pursues them into the Indian Ocean, beyond the range of RAF air support from South Africa. It is only then that the French fleet, thought to still be en route from the Atlantic, strikes from the west. The English fleet is outnumbered and outflanked on two sides. The remaining RN battleships make a last, desperate assault in both directions to buy the carriers time to escape north. After the Franco-Soviet fleet finishes off the enemy battleships at close range, they turn north to pursue the carriers. After a days-long running battle, the English carriers arrive in India, where they remain, safe but useless.
Meanwhile, both sides' militaries prepare for the Battle of Britain that will surely follow. The French brown water navy returns home, where several joint Franco-Soviet dress rehearsals are conducted. The RAF fights tooth-and-nail over southern England, but is forced to withdraw to disbursed airfields in the North, giving the Franco-Soviet Air Forces local dominance over the invasion area. After leaving sufficient firepower in the Indian Ocean to keep the surviving English carriers bottled up, the Franco-Soviet fleet returns to the Atlantic, where it launches raid after raid on the disbursed RAF forces. The reconstituted British Army digs in along the southern coast.
Finally, the long-awaited and dreaded day comes as entire corps of Soviet paratroopers descend on England, and the first waves of French Marines storm ashore. After a long, slow grind northward, the Franco-Soviet forces seize London. Organized resistance begins to crumble, and the invaders reach as far as Edinburgh before English forces capitulate. His Majesty's Government, shortly before capitulation, puts
Operation London Bridge into effect. The Royal Family and key members of government are flown out of the country, while the remnants of the British Army and hundreds of thousands of civilians escape on any vessel, military or civilian, capable of sailing in the open ocean. After meeting in Iceland to refuel, the motley fleet continues to Canada, then the Panama Canal, then Australia, then finally to India, the new seat of the British Empire in Exile, from where the Free British Forces wait for the day in which the free world will join them in the liberation of their island.