The current form of the flag of Slovakia was adopted by Slovakia’s Constitution, which came into force on 3 September 1992.
The Slovakia flag, in common with other Slavic nations, uses the white, blue and red colours.
Slovakia’s flag in its current form (but with another coat of arms on it or without any arms) can be dated back to the revolutionary year 1848 (see: The Revolutions of 1848 in the Habsburg areas). It was also used semi-officially in Czechoslovakia before World War II, by the Slovak Republic during WWII, and finally adopted (without the coat of arms) on 1 March 1990 as the flag of the Slovak Republic within Czechoslovakia. The coat of arms was added on 3 September 1992 and a special law describing the details of the flag followed in February 1993.
The blue triangle in the current flag of the Czech Republic, with which Slovakia formed Czechoslovakia up to the dissolution of Czechoslovakia, was taken over from the blue strip of Slovakia’s flag in 1920 into the flag of Czechoslovakia. The flag of Czechoslovakia was taken over by the Czech Republic in late 1992 in direct violation of the 1992 Act on the Division of Czechoslovakia explicitly forbidding state symbols to be used by the two successor states.
Since the Slovak flag without the coat of arms is identical to that of the modern flag of Russia and it can also be compared to the modern flag of Slovenia, the Constitution of Slovakia added the national coat of arms in September 1992.
The Double cross of the flag was inspired from Hungary’s coat of arms, which is composed by this same cross and the three mountains. This symbol was used by the Kings of the House of Arpad in the Medieval Times, and it passed to be part of the current monarch coat of arms from the 13th century to nowadays. Wikipedia