Argentina's flag was designed by Manuel Belgrano in 1812.

When Napoleon took over Spain in 1810, most american colonies raised against his regime, claiming they would only recognize Fernando VII as king, who was a prisoner of Napoleon.

So, when Belgrano created his flag, the government in Buenos Aires did not want him to raise it, since they were still pretending to be loyals to the (imprisoned) king, and a new flag would point out very clearly that they were claiming at Independence.
It is said that the colours of the flag resemble the colours of the Bourbon house, which is the Royal family of Fernando VII (Also, of current Spain's king, Juan Carlos).

Around 1818-1820 an Argentine fleet leaded by Hipólito Bouchard fought the Spanish along the Pacific coast of Central America, and that is a reason for so many Central American flags (Nicaragua, El Salvador, Honduras) resembling Argentine flag.

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