This was definitely a clusterfuck but one part needs clarifying, which shows how complex and nuanced the situation was.
When the British took over the former Ottoman territories, these covered a huge area, including modern Iraq, Jordan, Palestine and possibly parts of modern-day Saudi Arabia. These were not independent states at the time.
The McMahon letters of 1915 were deliberately ambiguous and there was a crucial issue in the translation of the English to the Arabic, which seemed to accede to the Arab demands, but that was never the intention. The term 'Palestine' was a Western one, but not an Ottoman one, and there was no administrative territory of that name under the Ottomans. There was reference to territories west of Damascus being excluded as "they were not exclusively Arab". This could mean places like the Galilee, where there was a significant Jewish presence, or it might not. We'll never know. But the McMahon letters were deeply cynical. The intention was to encourage an Arab front against the Turks (which it did).
The Sykes-Picot agreement, conducted as secret protocols, divided the area up into British & French spheres of influence but parts of what was known as Palestine (including Jerusalem) were intended to be under some sort of international control.
Then of course there was the Balfour Declaration is 1917 but the published version was a very-much watered-down version of the original. The revised version only talked about establishing a 'homeland' for the Jewish people, rather than the original request for the whole of the Palestine area to be exclusively Jewish. The original also requested unrestricted Jewish immigration, which was left out of the published version, which also talked about protection of the rights of Arabs. The intention was, on the surface, noble but obviously led to the present-day situation.
Incidentally, one of the prime movers in the Balfour Declaration was Charles Dreyfus, who set up Clayton Aniline where the CFA now stands, and who introduced Weizmann to Balfour.
No one really understands the reasoning behind the Balfour Declaration. One theory is that it suited the British to encourage its Jewish population to emigrate. Just a few years earlier, anti-immigration rhetoric (plus ca change) had been aimed at the wave of Jewish immigration from Eastern Europe. Therefore Palestine could have been the Rwanda equivalent. Another is the desire to keep the French out. So you could certainly argue that the BD was yet another cynical manoeuvre.
One the war had ended, the territory was parcelled up, with parts of it given to friendly Arab leaders, without much regard for tribal or territorial integrity, with what's now known as Palestine left under British control from 1923. All attempts to reach a negotiated settlement failed, ultimately leading to us handing back the mandate to the newly-created UN after WWII and the partition of the territory in 1948.