When the world war i broke out, the united states declared its policy of neutrality. was the united states ever neutral in the conflict, and if so, when did it change to a policy of favoring the allies?

The US was never actually 'neutral.' They didn't officially pick a side in the aspect at first but they did sell ammunition which was providing ammo to the Allies. Thus by doing so, they chose to help the Allies, picking a side.

On this day in 1914, as World War I erupted across Europe, President Woodrow Wilson declared that the United States would remain “impartial in thought as well as in action.” ... In February 1915, Germany declared unrestricted warfare against all ships, neutral or not, that entered the war zone around Britain.

the emancipation proclamation was the aim of the war change to include the freeing of slaves in addition to preserving the union. the 13th amendment, which abolished slavery in the u.s. was passed on december 6th 1865.

as promised, lincoln waited to unveil the proclamation until he could do so with success from the union military advance. on september 22nd 1862, after a victory at antietam, he publicly announced a preliminary emancipation proclamation, declaring all slaves free in the rebellious states as of january 1st 1863.