The Jews accepted the UN partition, although some did hope to adjust the borders. The Palestinians rejected it and chose war instead. The surrounding Arab states were preparing for war the whole time, intending to take land for their countries.
The United States indirectly helped the Arabs by suspending the importation of arms into the region The UK helped by selling arms to Arab militias and blocking additional Jewish migration. Although, the US was the first to recognize Israel when they declared independence.
The different countries and groups against the Jews had different interests and different levels of commitment.
Many of the villages that the Jews forced to evacuate, were supporting Arab militias attacking Jews, so having them evacuate was a defensive strategy to not leave a population of enemy combatants scattered around the area. The combatants blended back into their villages, so it’s not like just the combatants could be arrested. Some of these combatants didn’t just fight Jewish armed forces, but attacked civilians.
Overtime, ideas of expelling all Arabs had been advocated for, but the actual expulsions were defensive military operations. Such ideas were often driven by the determination of the Arabs to expel and kill the Jews, so, the thinking was: if you can’t be peaceful and accept us here, then we will kick you out. Nevertheless, the actual expulsions were focused on villages supporting violent militias in the midst of war.
We really don’t know how many expulsions were completely unjustified and how many were battlefield necessities.
Jewish refugees, many from the holocaust, moved into the abandoned settlements.
Many Arabs fled on their own, encouraged to do so by some Arab leaders.
There was two steps to the war. First, the civil war between the Jews and local Arab militias, then, the Jews against the invading Arab states.