"...increasing synthesis between the worldview of Paleologue and that of Castelnau, who by early 1917 had been War Minister for close to a decade and whose influence over the Army rivaled Boulanger's at the height of his powers; Poincaré, recalling how Boulanger had loomed over every Cabinet of the late 19th century for a decade and a half like a saber suspended by string, was uninterested in testing what could come of the endlessly ambitious vicomte being denied his due.
Ironically, Castelnau had few if any designs on the Premiership. He was already the bete noire of the opposition bloc, despised for his fervent Catholicism (even by the standards of Late Empire France) by the secularist radicals, denounced as an aristocratic reactionary by the socialists, and blamed for the imposition of the service term of three years for the French Army in order to keep the peacetime forces at par with those of Germany. [1] As such, due to both his age (he was in his late sixties when the Central European War broke out) and this strong unpopularity with the French public, Castelnau exercised his vanity not in the pursuit of grander office but the formation of a fiefdom around the extant office he already held, a pursuit of "le petit-royaume," as it came to be known in the years of the French State, where Cabinet officials and junior ministers served for years if not decades on end with few ambitions other than to leverage their portfolio into personal riches and create a motte-and-bailey over what they considered their prerogatives.
As 1917 dawned, it became abundantly clear to French policymakers that since the turn of the century, accelerated by the deaths of the Napoleon IV and Boulanger, French foreign affairs had been a dismal failure bordering on disastrous. While the Little Eagle had been mercurial, he had also been dynamic and hugely popular with both the French street (even begrudgingly respected by republicans) and foreign courts; for as indecisive as Boulanger had proven himself to be, he had nonetheless projected force and few had been willing to test whether his belligerency was merely bluster. Napoleon V and Poincaré were no such figures, and nobody seemed to be waiting in the wings who could act as such either; the rising personalities of the Late Empire were rather socialists such as Jaures or Blum, social-democrats such as Briand, or radicals such as Doumer and Doumerge. Politically, it seemed the National Bloc's time was closing in on its end, and if a political movement that prided itself on military strength was in decline, what did that bode for France when pacifist, internationalist and Marxist forces came to bear next?
Such malaise for the long-dominant droite was perhaps inevitable though with the course of French policy. The Boxer War had seen France deepen her hold over Kwang-chou-wan and earn Chefou in Shandong, but German, American and British influence had rapidly spread out from their concessions in the Southeast and the Yangtze Valleys to keep French influence cabined in the far south, where their hold over Yunnan's local officials was rapidly fraying - and such restiveness now spread to Vietnam, particularly in the borderlands. Korea, once a pseudo-protectorate of France, had never formally left the French fold but the court in Seoul clearly favored Russian, Japanese and American influence and the French resident-general was a token figure largely confined to the Busan concession as "merchants" and "missionaries" covertly carved out their niches in Korean affairs under Paris' nose. [2] Africa had seen British designs on the Indian Ocean coast realized and Germany sandwiching holdings in the Congo and Dahomey with their possessions, while effectively from their camps in Kamerun projecting their claims deep into the Ubangi-Shari, essentially foreclosing on French hopes to penetrate the Great Lakes region and ending the dream of the Dakar-Djibouti rail line that would stitch French possessions in northwest and northeast Africa together across the lower Sahel and deep interior of the continent.
The colonial setbacks, so soon after the hugely prestigious winning of such prizes at the end of the century, paled in comparison to the two most critical projects of French foreign policy over the last half century, two of the few consistencies in the fluid geopolitics of the Belle Epoque. The first was France's maneuvers in the New World - its longstanding alliance with Mexico, its recognition and economic support of the Confederate States, and its encouragement of a Francophile line in the Imperial Brazilian elite, all an effort that created the blueprint upon which the Bloc Sud of the Great American War could be built. This grand though largely unofficial and informal project finally collapsed with the exit of Mexico from the war in autumn of 1915, the peace between Brazil and Argentina in February of 1916 and between Rio de Janeiro and Philadelphia eight months later, and then with the near-collapse of the Confederate States as a viable polity in November, concluding with the humiliations exerted upon Richmond at the Mount Vernon Congress in early 1917. The hegemony of the United States in the Americas, long checked by French underwriting of her smaller but collectively fearsome rivals, was now undeterred, and few in the incoming administration of President Elihu Root forgot that Paris had been the most consistent champion of the Bloc Sud for the duration of the Great American War.
In the same month that the American endeavor came crashing down, France's more important security policy started to appear to be unraveling as the November Crisis over the Hungarian government and electoral franchise brought Austria-Hungary to the brink of internal conflict just as Franz Josef I passed away. The death of the Austrian Emperor was the more crucial piece of the immediate crisis, as it took away a figure whom French diplomats had come to see as a safe harbor in any storm, a reliable figure who had reigned since the same year Napoleon III, 68 years prior, took power in the wake of the June Days, longer than most French policymakers had been alive. Though not as absolutist as his detractors claimed, the "Alte Herr" had nonetheless been a firm-handed autocrat who brooked no fools and was predictable in how be managed the fractious politics of his polyethnic empire, and the advice he had dispensed to Emperors and Prime Ministers alike had been invaluable. The Rock of Catholicism, as Napoleon V eulogized him in Vienna at his funeral, was gone - a steady hand upon whom France had depended a great deal.
In his place came a murkier figure. His nephew Ferdinand II was seen as highly capable and very intelligent, but was not particularly well-liked in Paris. His hunting habit was considered extreme and excessive even by the standards of the day, and his marital strife and long-running affair with a Czech noblewoman who had borne him as many children out of wedlock as his estranged Saxon-born wife had borne him within it. He was thought to be considerably more open-minded about Austrian security arrangements than his uncle and was keen on a rapprochement with Germany, and the Quai d'Orsay was concerned he was open to throwing France under the bus to accomplish it. Though he spent most of his time in Prague, he was not known to hold Slavic culture in particular high regard, but saved most of his contempt for Hungarians, loathing not only the nationalists who chafed against Habsburg rule but also the ostensibly pro-Vienna magnates whom he held responsible for the various political divisions that had badly reduced Austrian military readiness. That Franz Josef had died so shortly before the negotiations to renew the 1867 Austro-Hungarian Compromise was an unmitigated disaster, and Hungary looked poised to erupt as an Emperor who despised half his realm looked to make his mark.
This confluence of events in the mid-1910s left French policy adrift and greatly empowered the militants such as Castelnau, though that oversimplified the situation greatly, as where Poincaré was there were never moderates close by. The use of French finance as a foreign policy tool now became a military tool as well, as the February 1917 budget dramatically raised military spending, nearly doubling the outlay on the Army by 1919 and expanding the Navy's budget, primarily on additional battlecruisers rather than dreadnoughts, by sixty percent; the allocation for air power, based on observations of its lethal effectiveness in the Great American War, was tripled. Socialists and radicals were appalled and voted in large numbers against the measure, but they narrowly passed, and the 1917 budget came in hindsight to be seen for what it was - a declaration of French belligerency to the rest of Europe and the world, that of a paranoid and insecure Paris wanting to aggressively show its strength.
Other powers noticed; both Germany and Italy responded later in the year with their own budget increases, with it becoming a major electoral issue in the latter as Italians went to the polls in the fall of 1917 under expanded suffrage. Belgium, too, dramatically increased its military budget and extended her terms of conscription, but also fully standardized its military kit to match those of France for "interoperability," taking advantage of the largesse of French loans to an already affluent Treasury in Brussels. An arms race on the continent was thus, thanks to a sharp decline in France's geostrategic position, underway, and unlike prior arms crises, this one would not have a peaceful conclusion..."
- La Politique Mondiale: Poincaré, France and the Waltz of the Great Powers
[1] It should be noted that despite all this, Castelnau was iOTL a staunch republican and a fierce opponent of Petain and the Vichy regime, correctly regarding them as opportunists and traitors.
[2] I realized I never really established how France's protectorate in Korea dwindled to being basically just a scrap of paper, and this is sort of how. The post-1872 and post-1885 treaties never went out of effect, it's just rather that France has struggled to enforce its position there, at all.