9 February 1990
The acceptance of the Namibian constitution on 9 February 1990 established the Republic of Namibia.  The independence of Namibia can be directly traced to an accord agreed upon by Cuba, Angola and South Africa. The reason that these particular parties were instrumental in Namibia's independence is that Namibia was a protectorate of South Africa from 1920 until 1990. This meant that South African laws applied to Namibia. These included the laws designed to entrench White minority rule. After years of internal uprisings, which culminated in the Battle of Cuito Carnivale, and dealt a deathblow to the Apartheid regime's habits of engaging in cross border conflict, the parties involved came to an agreement to cease hostilities on the condition that Namibia attained independence. Namibia's constitution, in its day was seen as most progressive.
References

WIPO, Namibia: The Constitution of The Republic of Namibia, from World Intellectual Property Organisation, [online], Available at www.wipo.int [Accessed: 17 January 2014]|SAHO, The Battle of Cuito Carnevale, from South African History Online, [online], Available at sahistory.org.za [Accessed: 05 February 2010]