Why was the manhattan project kept top secret?
the united states did not want great britain or france to know that it had nuclear technology.
no one was sure if the atom bomb would actually ever work and they didn't want negative feedback.
the allies were afraid that the technology would be stolen by spies belonging to germany or japan.
the united states was developing the technology illegally and with stolen technology and materials.

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.