The Maronites sharing power with two more parts of state instead of just the Druzes might seem strange. But before, in the 19th century, the situation largely favoured the Druzes, especially in the so called mixed areas of the two qaim-mayqamam. The Druzes and Maronites also didn't share power as much as they were forced to cooperate by the Ottomans and told what to do by politicians like Metternich or incited to rebel by French and British influences. That is just the pinnacle of the unease and tensed situation throughout the entire century, with bitter rivalries between all groups. And these were more than just the two mentioned in the question from the get-go.
Thus the concept for a Greater Lebanon had the support from the Maronites at that point as the own private Idaho idea with Ottoman Empire gave way to dominating power politics in a more independent region, albeit under French protection. That meant a few things, incorporating French wishes and interests, securing a manageable political structure, that would be able to function food-producing wise and economically. Incorporating a few less inhabited but fertile region considered "Arab" to dominate was just the recipe for that.
"All groups" as there were already quite a few of them more than just two. The arrangement for the latter half of the century was set out to be
The mutasarrifiya built on multicommunal representation in the qaim-maqamate, compelling collaboration to operate a structure in which sects had defined shares. From the outset, the administrative council had the right to veto tax increases and oversee appointments, a far-reaching advance on the advisory function of the tribunals of the qaim-maqams. Electoral representation and rough demographic weighting of communal membership were also innovations. Governors’ councils in regular Ottoman provinces did not have the rights of the Mount Lebanon council and were appointed, not elected.
Of twelve councilors, four were Maronite, three Druze, and two Orthodox, with one each for Greek Catholics, Shia, and Sunnis. This made a Christian majority of seven, but in formal terms a Maronite minority despite the Maronite population majority. A two-stage electoral process became re ned over several decades, with secret balloting introduced in 1907. Adult male village and town residents elected headmen who met in six districts and the town of Zahle to elect one or more council members assigned to each of these seven constituencies. For example, the Shuf contributed one Druze member while the Matn had four seats — Maronite, Druze, Orthodox, and Shia. All headmen voted for all district members, regardless of sect. Maronite votes could thereby influence non-Maronite outcomes, mitigating Maronite under-representation. Elections for one-third of council seats took place every two years. The mutasarrifiya introduced a core element of modern confessional democracy—multimember, multisectarian constituencies.
This arrangement is then said for the 1860s and 70s at lest to yield "Maronite acquiescence, Druze reintegration, and sectarian reconciliation." And within Ottoman frameworks of administration, from which all groups in the area wanted to distance themselves and their local privileges, the interplay grew ever more complex:
[…] Beirut’s local merchant and professional class kept a firm grip on the city’s economy, restricting Europeans. Christians dominated long-distance trade, banking, insurance, and silk processing. Sunni Muslims continued to specialize in eastern Mediterranean commerce. Sectarian incidents between Christians and Muslims increased on the streets with Christian in-migration. Christian and Muslim street leaders, or qabadays, emerged among shopkeepers, artisans, and laborers in response to confessional friction, but they had an interest in being useful to authority and containing conflict.
Overall, the prosperous expanding elite and middle class cohered reasonably. They drew in muqata’ji families of the mountain, Maronite and Druze, who found alignment with urban merchants essential to maintaining their lifestyles. The Khazens took loans from the Lahouds and Thabits, the Abi Lamas from the Asfars, and the Arslans from the Baz family. The Sunni Bayhums established a variety of cross-sectarian connections: financial partnerships with the Junblats and other Druze, marriage with Maronite Shihabs, and joint land purchases with the Orthodox Bustroses.
The direct hinterland of Beirut may have been Maronite dominated but the fringes to be incorporated into Greater Lebanon were already pouring into the urbanising and demographically exploding urban centres.
Shortly before the war the Ottomans started an attempt to unify – or turkify –Syrian provinces which accelerated when the war started. While the Ottomans wanted to suppress local peculiarities the Allies furthered them – but British interest immediately clashed with French ambitions.
Facing the prospect of British seizure of the Levant after October 1914, France quickly staked its claim. For the allies, the war changed everything; for them the Ottomans were finished. Between December 1915 and May 1916, French and British negotiators François Georges-Picot and Mark Sykes hammered out an arrangement for the Ottoman Arab provinces. Picot had been France’s consul-general in Beirut and envisaged French hegemony in the Levant while the British took Iraq. This was too much for the British given that they bore the military burden in the Middle East and needed to encourage their Arab ally, Sharif Husayn of Mecca and the Hijaz, into rebellion against the Ottomans. In a compromise, Picot obtained agreement to French command of a Mediterranean coastal strip north of Palestine to be ruled from Beirut, notably an expanded Mount Lebanon. The interior beyond the Biqa and the Orontes River would be an Arab state, with the areas from Damascus north under French “influence.” For the British, nothing here contradicted their correspondence with Sharif Husayn, whom they had told from the outset that France would have preeminence on the northern Mediterranean coast of Syria.
For the people of the effectively defunct provinces of Beirut and Mount Lebanon, 1916 was the year in which decentralism flashed into concepts of Arab independence, mountain independence, and Maronite assertion under French protection.
But the Maronites were also hit hardest by the war. Grain being confiscated, a local locust plague and general hardship led many to conclude that with half of them on Mount Lebanon dead from hunger they needed a state that could provide for them better. For them that firmly excluded Arab leadership and favoured French protection.
In early 1919, France toyed with the idea of Mount Lebanon as a component of a Syrian Arab kingdom, provided Faysal accepted French tutelage. Alarmed Maronites in both the church and the administrative council campaigned strenuously through 1919 for French commitment to an enlarged Mount Lebanon, and Patriarch al-Huwayyik lobbied for French rule. The administrative council, including the patriarch’s brother, wavered between French protection and an arrangement with Faysal, though the objective of a Greater Lebanon was the same.
By late 1919, Maronite activism, support from French lobbies, and the hostility of Faysal’s Syrian Arab nationalist colleagues in Damascus brought France and its Maronite friends into concordance. The American King-Crane fact-finding mission of July 1919 had a galvanizing effect. It highlighted Arab nationalism and rejection of France among the Sunni Muslims of Beirut, Tripoli, and Sidon, as well as a large part of the Shia, Druze, and Orthodox Christian communities of any Greater Lebanon. Conversely, Patriarch al-Huwayyik’s personal mission to Paris and the Versailles peace conference between August and October 1919 raised Maronite stocks. The patriarch also enlisted Vatican support. Meanwhile, British evacuation from the northern Levant in September 1919 le the French in Beirut facing Faysal in Damascus.
In November 1919, Premier Clemenceau gave written French endorsement for a Lebanon separate from the Syrian interior. He appointed the energetic General Henri Gouraud to succeed Georges-Picot as high commissioner in Beirut. In December, Clemenceau extracted Faysal’s agreement to a Lebanon adding Sidon, Jabal Amil, Wadi al-Taym, and the western side of the Biqa to the mountain and Beirut. This, however, fell short of Maronite aspirations in the Biqa and the north.
Maronite submissions to France and the Versailles conference reached back to a French map of a potential Greater Lebanon drawn under the auspices of General Charles de Beaufort d’Hautpoul in 1861. Beaufort was the commander of the French expeditionary force sent a er the 1860 disturbances and a proponent of France’s mission to Lebanon’s Catholics. Maronite leaders, especially Patriarch al-Huwayyik, shrewdly conflated a new Lebanon with ancient Phoenicia to highlight a unique personality. The Phoenician allusion derived from a European fashion for romanticizing antiquity, for example the 1864 book Mission de Phénicie of Ernest Renan.
Among Maronite publicists, the demand for an enlarged Lebanon as a Catholic Christian homeland began with the tracts of Philippe and Farid al-Khazen and Bulos Nujaym between 1900 and 1908. Like Arabism, it gained ground after the 1908 Istanbul coup and developed through World War I into a Lebanese Christian nationalism fervently endorsed by exiles and emigrants from Egypt to Paris to Latin America. Its leading thinker a er 1918 was the young Latin Catholic banker and journalist Michel Chiha, through his mother, a member of the influential Greek Catholic Far’un family. Chiha gave “Lebanism” a pluralist veneer appropriate for inclusion of non-Catholics and non-Christians. There was no doubt, however, that Maronites and other Catholics would dominate.
Among a wider public, the resurrection of Phoenicia and the concept of a distinctive Lebanon received stimulus from the literary output of Jibran Khalil Jibran, who fashioned a dreamy image of Mount Lebanon. Jibran, from a humble Maronite family in Bsharri, lived continuously in the United States from 1902, but his Arabic evocation of a spiritualized mountain people and landscape had growing popular impact in the last Ottoman years. He also drew out feelings of oppression, conflict with established religion, and interpersonal tensions. These had resonance for a multi-sectarian Lebanon. Jibran’s combination of Christian ambience with outreach to Muslims reinforced Chiha, even if his appeal to Muslims was patchy. Incorporation of Jibran in school curricula a er the 1920s helped make him by far Lebanon’s most in uential writer, apart from his international reputation for his post-1920 output in English.
“Lebanist” Greater Lebanon comprised the Lebanon and Anti-Lebanon mountains, with the Biqa Valley between them and the associated coast and ports. This was a clear unit on any map — as reflected in the political cartography of General Beaufort. Patriarch al-Huwayyik had most of it from Clemenceau and Faysal at the end of 1919. The wartime famine and Maronite outposts determined him to complete acquisition of the Biqa, the principal food-producing area. Military activity against the French in the First months of 1920 on the Biqa front between General Gouraud’s forces and the Damascus-based Arab nationalists ensured Gouraud’s backing. Gouraud favored a boundary well into the Anti-Lebanon Range.
To the north, it was unthinkable not to incorporate all of Mount Lebanon, bringing in the Christians of the Akkar. Tripoli proved more controversial. Gouraud’s secretary and advisor Robert de Caix wanted a Greater Lebanon with a secure Christian majority and opposed inclusion of Tripoli’s hostile Arab nationalists and Sunni Muslims. For the Maronite church, however, Tripoli was their historical outlet to the Mediterranean. Gouraud went with the church.
To the south, the interaction between France and Britain took precedence. British commitment to a “national home” for the Jewish people in Palestine in the Balfour declaration of November 1917 determined the French to keep Britain and the Zionists distant from Beirut. The French agreed to British control of the Upper Jordan Valley, reflecting Jewish colonization near Lake Hula, but otherwise took their stand at Ra’s Naqura, the natural barrier where a nger of the Galilee hills meets the sea in an impressive headland halfway between Acre and Tyre.
–– William Harris: "Lebanon. A History 600–2011", Oxford University Press: Oxford, New York, 2012.