Q: Using both definite and indefinite article for the same noun Is it okay to use both "a" and "the" articles in relation to the same noun? For example,

Finally, King George III came into power. A very long and the last
  peaceful period of the history of Bakhardia started.


A: Yes, there is a parallel structure implied here:

A very long peaceful period of the history of Bakhardia and the last peaceful period of the history of Bakhardia started.

Is the implied meaning, and in this full version, you can see that "A" and "The" have been used correctly. But the repetition is unnecessary if a parallel structure is used, which gives your sentence.
