He favoured a United States of Europe, and participated in drafting the constitution for the Fifth Republic, but resigned from government in 1962 after disagreement with President de Gaulle over changes to the electoral system.

Although he was first elected as part of the conservative "Blue Horizon"[8] bloc in 1919, Reynaud shortly thereafter switched his allegiance to the centre-right Democratic Republican Alliance party, later becoming its vice-president.

In the 1920s, Reynaud developed a reputation for laxity on German reparations, at a time when many in the French government backed harsher terms for Germany.

Reynaud backed a strong alliance with the United Kingdom and, unlike many others on the French Right, better relations with the Soviet Union as a counterweight against the Germans.

[9]: 517 Reynaud held several cabinet posts in the early 1930s, but he clashed with members of his party after 1932 over French foreign and defense policy.

Reynaud was a supporter of Charles de Gaulle's theories of mechanized warfare in contrast to the static defense doctrines that were in vogue among many of his countrymen, symbolized by the Maginot Line.

Reynaud publicly made his case, and in response Flandin pamphleted Paris in order to pressure the government to agree to Hitler's demands.

He endorsed radically liberal economic policies in order to draw France's economy out of stagnation, centered on a massive program of deregulation, including the elimination of the forty-hour work week.

The meeting culminated in a declaration with British Prime Minister Neville Chamberlain, stating that neither country would seek a separate peace.

Following Britain's decision to withdraw on 26 April, Reynaud travelled to London to personally urge the British to continue their fight in Norway.

The dire state of French equipment and morale was underscored by a postcard found on the body of an officer who had committed suicide in Le Mans.

This marked the beginning of the British May 1940 War Cabinet Crisis, where Halifax supported what was known as "the Reynaud Option"—exploring negotiations with the Italians for acceptable peace terms, potentially involving concessions in the Mediterranean.

[18]: 223–227 In early June, Charles de Gaulle, whom Reynaud had long supported and one of the few French commanders to achieve success against the Germans in May 1940, was promoted to brigadier general and appointed undersecretary of war.

[19]: 138–142  Italy entered the war on 10 June, and on the same day, General Weygand, the Commander-in-Chief, stormed into Reynaud's office and demanded an armistice.

By the Cabinet meeting on the evening of 12 June, it was evident that a significant faction was pushing for an armistice, leading to the decision to relocate the government to Bordeaux rather than retreat to a fortified Brittany.

[20]: 197–198 During the subsequent Anglo-French conference in Tours on 13 June, Reynaud requested to be released from the agreement he had made with Prime Minister Neville Chamberlain in March 1940, so that France could pursue an armistice.

That evening, Pétain, who strongly supported Weygand's call for an armistice, announced his intention to remain in France to share the suffering of the French people and begin a national renewal.

[22]: 82–86  Churchill's telegram, also received on 16 June, proposed an armistice if the French fleet was moved to British ports—a condition unacceptable to Darlan, who feared it would leave France defenseless.

[22]: 82–86  That afternoon, de Gaulle was in London discussing a proposed Franco-British Union—a plan hastily put together by Churchill and his advisers to support Reynaud against those favouring an armistice and to keep France, especially its naval fleet, in the war alongside Britain.

Time was critical, and de Gaulle dictated the "Declaration of Union" to Reynaud over the phone, enabling him to present it to his Cabinet that afternoon in an attempt to counter the armistice faction.

When the French Cabinet met in Bordeaux that afternoon, Reynaud presented the British union plan and, with Georges Mandel, declared his resolve to continue fighting.

Lebrun reluctantly accepted Reynaud's resignation, and the French government, at this critical moment, fell into the hands of Pétain and those advocating for an armistice and collaboration with the German invader.

[20]: 204–205 Julian Jackson notes that Reynaud felt a profound sense of guilt for 20 years over allowing Pétain to come to power and offered increasingly convoluted explanations for the events that transpired.

Despite Reynaud's own fighting spirit and a brief moment of indecision on 26 May, he regretted not being able to emulate Clemenceau, France's great wartime Prime Minister from 1917 to 1918, and he never forgave himself for failing to be another de Gaulle.

There were suggestions that Reynaud might have mustered a Cabinet majority for continuing the fight, but he later argued that he could not counter the political influence of the "softs", particularly Pétain and Weygand, France's leading military figures.

[20]: 206–207  De Gaulle returned to Bordeaux around 10 pm on 16 June and visited Reynaud, who still hoped to relocate to North Africa and declined to go to London.

De Gaulle, accompanied by Edward Spears, flew to London at 9 a.m. on 17 June and made his famous broadcast the following day, declaring his intent to continue the fight.

On 28 June, while Reynaud was driving their Renault Juvaquatre, the car veered off the road and crashed into a plane tree at La Peyrade, near Sète.

[30] During the Battle for Castle Itter on 5 May 1945, Major Josef Gangl, a Wehrmacht officer who had joined the anti-Nazi Austrian resistance, was killed by a sniper's bullet while attempting to protect Reynaud and other prisoners.

After she had borne him two children, the marriage failed, and when Reynaud and his wife separated in 1938, Hélène de Portes was his mistress until her death in the road accident at Frontignan in 1940.