Obviously the best time to stop the expansionist ambitions of the dictatorships is as early as possible; before they got used to the idea that the decadent democracies would simply acquiesce to anything they did. For Hitler that is in March 1936, when he ordered the remilitarization of the Rhineland.
Hitler’s remilitarization of the Rhineland was in direct violation of the Treaty of Versailles and the Locarno Pact signed in 1925. As such he had committed a hostile act and one that threatened the basic assumptions of France’s national security; France had full legal justification to use military force to expel the German armed forces from the Rhineland and moreover, a very real strategic reason to oppose the presence of German troops west of the Rhine.
Had they wished, the French had the forces to do easily remove the Germans from the Rhineland; the Wehrmacht had sent nineteen infantry battalions to occupy the Rhineland, a total of 32,000 troops out of overall army strength of 250,000. They had little in the way of artillery, no heavy artillery at all and the only armour they had was the Panzer Mk I, which was armed with two machine guns, had no more than 13 mm of armour and shouldn’t have even been considered to be a tank at all, it was really just the German equivalent of the British Bren gun carrier. The ground forces were backed by a Luftwaffe that had only ten available combat aircraft.
France on the other hand, without calling up reserves, had 320,000 troops available in mainland France and another 100,000 native troops in North Africa that could be moved to the mainland on short notice. These were well equipped with artillery and heavy artillery in abundance and a doctrine that believed that artillery could dominate the battlefield. The French also had a large number of tanks to support their infantry. The French air force was also one of the largest in the world and had the most modern and fastest fighter and bomber aircraft in the world. If there were a battle for the Rhineland it would have been decidedly one sided even if the French hadn’t mobilized their huge army reserve.
There would not have been a battle for the Rhineland even if the French had reacted; Hitler was bluffing and had given orders that no resistance was to be offered to any French reaction. At the first sign of trouble his forces were to march out of the Rhineland as quickly as they had marched in; Germany simply lacked the strength for a fight. Conscription had only been reintroduced the year before, in that year the army had doubled in size, something no army could do with any serious chance of maintaining quality. Rearmament had barely started and the Luftwaffe had just celebrated its first birthday and consisted of mostly training aircraft (hence they only had ten actual combat aircraft available to support the reoccupation.)
Had they decided to do so the French could have marched into the Rhineland in strength, unopposed and, because of the treaties of Versailles and Locarno, fully within their rights to do so and to remain until the situation in Germany improved, depriving Hitler of the mining and industry of the entire Rhineland including the Saar, and exposing the Ruhr industrial heartland of Germany to bombardment should fighting breakout.
If that had happened, the mystique of Fuhrer infallibility that started then and grew with each successive back-down by the western powers in the years that followed would have been stillborn. Hitler would have faced more resistance internally to his plans for war. Even if the bumbling German opposition to Hitler had never improved, Hitler would still have been faced with French control of the Rhineland buffer, the broad obstacle of the Rhine itself, in some places half a kilometre wide and swiftly flowing, as well as ground east of the Rhine itself.
Unfortunately the French did nothing and for this we have primarily one man to blame: General Maurice Gamelin, commander of the French Army. In February 1936, Gamelin warned the French Cabinet that such a move by the Germans was possible and had been told to prepare a contingency plan accordingly. When the time came a month later, Gamelin informed Prime Minister Sarraut and his amazed cabinet that the French army was totally unprepared for combat and that it would require the mobilisation of the entire army, all 3.5 million men! It is unfortunate for posterity that no-one in the room thought to question this truly insane assessment. Such a mobilisation was of course totally out of the question; France was only six weeks away from an election and mobilisation would have been hugely unpopular with the French electorate, it would have also been cripplingly expensive.
The fact that Gamelin had had prior knowledge of a possible German move, had been told to make preparations accordingly and then announced that nothing could be done short of full mobilisation, should have been an opportunity to retire him for incompetency and irresponsibility. In fact it would have been the perfect opportunity for a clean sweep of the French high command, most of who were already in their mid-sixties, some in their seventies! Instead, Gamelin remained in command. He was still in command and still insisting that nothing could be done when the Panzers poured into France four years later.
Hitler was reasonably confident that the British and French would not oppose his move into the Rhineland because they’d done nothing to oppose Mussolini when he’d invaded Abyssinia the year before.
Instead of taking action against the Italians and aiding the Abyssinians, the League of Nations, primarily under the guidance of the British and French, decided to treat victim and assailant the same: imposing an arms embargo on both countries! This didn’t hinder the Italians with their invasion; they manufactured their own tanks, and aircraft. All it did was make it even harder for Haile Selassie’s people to defend themselves. The League had been founded on the cornerstone of Collective Security, but faced with the first real challenge to the security of a member of the league, did nothing to help and, by introducing the arms embargo on both nations, actively hindered Abyssinia’s defence. Ironically, the only countries willing to supply arms to the Abyssinians in significant amounts were Nazi Germany and Imperial Japan!
The Italian invasion was only able to continue with the complicity of the British. Every Italian soldier sent and every bullet, tank, aircraft and bomb used on the Abyssinians, was sent there on a ship that had to pass through the British controlled Suez Canal. The British government chose not to stop Italian shipping using the canal because they feared that if they did so, Mussolini would invade Egypt with the large Italian army in Libya and simply seize the canal for himself. That the Royal navy was still the largest in the world and dwarfed the Regia Marina, doesn’t seem to have factored large in their deliberations, the idea of strengthening the British garrison in Egypt doesn’t seem to have occurred to them either. The British went so far as to aid the Italians in their propaganda war; accepting the Italian claim that they were not using poison gas and the ludicrous Italian claim that photos of Abyssinian victims of Mustard Gas, were actually suffering from leprosy instead.
The League of Nations did impose economic sanctions on the Italians, but they did so late, ineffectively, and they were dropped as soon as the Italians captured Addis Ababa. The lesson drawn by all was that the democracies would not take any direct military action to stop aggression and any indirect action would be dropped as soon as the war was won.
Had the British closed the Suez Canal to Italian troop ships, which they had the full legal right to do so, Mussolini is unlikely to have done anything in response except shout curses from his balcony; Italy was struggling to invade an isolated African kingdom, let alone picking a fight against the largest empire in the world with the largest fleet in the world. Mussolini might have tried to supply the invasion by sending ships out the Straits of Gibraltar and around Africa, but they were already struggling as it was, their logistics probably would have collapsed under the added strain.
So, by actually taking part in the Collective Security of the League of Nations, the British and French would have made the smaller nations of Central and Eastern Europe more confident that the democracies could be relied on to help them, and made the Germans and Italians realise that there wouldn’t be any cheap victories. Doing so could have prevented World War Two and it would certainly have delayed it by several years.