Has any president declared a national emergency?

The National Emergency has been declared 3 times in India. 

• The first instance was between 1962 to 1968 during the India-China war, when “the security of India” was declared as being “threatened by external aggression”. 
• The second instance was in 1971 during the Indo-Pakistan war. V.V.Giri was the president at that time. 
• The third proclamation was between 1975 to 1977 under controversial circumstances of political instability. The emergency was declared on the basis of “internal disturbance”. Fakhruddin Ali Ahmed was the president during the third emergency.

Further Reading:

• Article 356 – President’s Rule
• 42nd Amendment of Indian Constitution