Here is a fascinating 10 minute clip from Youtube. It is a contemporary Italian documentary showing the highlights of 1 November 1950 when Pope Pius XII publicly declared the Dogma of the Assumption of the Virgin Mary:
""By the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory"
The names of the bishops attending the dogma celebration in 1950 are listed at the entrance of St. Peter's Basilica.
As Pope he had consulted first as to whether the Dogma should be defined. On 1st May 1946 he promulgated Deiparae Virginis Mariae which started the consultative process
On 1st November 1950 the Apostolic Constitution Munificentissimus Deus was promulgated defining the new Dogma
Interestingly, Munificentissimus Deus reports the popular acclaim and "nearly unanimous" approval of the contemporary bishops to the definition of the Dogma. As can be seen from the two writings by Pius XII the Assumption was not plucked out of the air by Pius XII and made an article of faith a bit like a medieval despot or monarch would. The belief in the Assumption was of very longstanding and had been prevalent throughout all parts of the Church
Why the definition of the Dogma by Pope Pius XII attracted such condemnation at a later date (which still prevails today) can only be ascribed to ignorance