In book 1 and 2, the fire nation was on the rampage to take over all the nations. At the end of book two, they finally took down Ba Sing Sae(spelling) the earth kingdom. After they took over Ba Sign Sae, they officially took over all the nations. In the beginning of book 3, you will see that all the tribes have been converted to fire nation and Ang is supposed to be dead. The crew is wearing fire nation clothe to hide out until the Solar eclipse comes so they can make a surprised attack.
Here you go, I forgot I had this link
http://www.tv-links.co.uk/listings/3/179