That depends how you define 'realistic'.
Hitler used 'salami tactics', with the French unwilling to go to war over what were individually minor escalations of German breaches of the Treaty of Versailles.
At each step, the probability of a rapid French success in enforcing the treaty without risking their armed forces becoming embroiled in a full scale war became less, and with memories of the Great War still fresh, it would have been political suicide for any French politician to start another war with Germany.
The tendency to think 'Even a remilitarised and belligerent Germany is never getting past the Maginot Line, so France herself isn't at risk, and while it's a bit hard on the Czechs to give their country to Germany, really, the Sudetenland is basically German, and they were a part of Austria-Hungary until a few years ago, so if Hitler wants it, it's better all around to let him have it' is understandable.
England can't really act without French approval, and war absolutely must be avoided at all costs.
Without a crystal ball to see the future, it strikes me as highly unrealistic that another course could have been sold to the people or the parliaments of either France or England.
Had it been, the French military would have crushed Hitler's unfinished armies like a bug - on paper.
But wars aren't fought on paper; If they were, Putin would have been in charge in Kyiv weeks ago.
Counterfactual history always has the benefit of going exactly the way its writers expect it to go.
I strongly suspect that France and England had the military strength to stop WWII in Europe by acting against Hitler earlier; Certainly the size, training, and equipment of the French forces appeared to be more than adequate to the task. But for their use in this way to be realistic, a very large number of people would have had to be far less averse to war than was actually the case. I would hesitate to call it 'realistic' for them to change their minds, given that they knew the past in horrific detail, and had no way to know the future.
There was a widespread belief that unstoppable air raids using chemical weapons as well as explosives and incendiaries might be inflicted upon London, Paris and other major cities within hours of war being declared. The absence of these very widely anticipated raids came as a somewhat embarrassing (though pleasant) surprise to many.
Indeed, the 'Phony war' period suggests strongly that none of the belligerents on either side was ready, willing and able to attack their enemies on land, even as late as April 1940, eight months after war was declared. There's a big difference between declaring war and actually invading your enemy's territory.
The 'Phony war' was a kind of halfway step between economic sanctions and full blown war, in which naval blockades attempted to do economic, rather than physical, damage to enemy nations.
