Rama Rajya indicates the rule of Shri Rama, as mentioned in the final Sarga of Yuddha Kanda of Ramayana. It narrates the happy and prosperous lives of the people in the rule of Shri Rama.
Shri M.K. Gandhi stated (19-9-1929) on his understanding of Rama Rajya, as follows;
“BY RAMA RAJYA I do not mean Hindu Raj. I mean by Ramarajya Divine Raj, the Kingdom of God. For me Rama and Rahim are one and the same deity. I acknowledge no other God but the one God of truth and righteousness.
Whether Rama of my imagination ever lived or not on this earth, the ancient ideal of Rama rajya is undoubtedly one of true democracy in which the meanest citizen could be sure of swift justice without an elaborate and costly procedure. Even the dog is described by the poet to have received justice under Ramarajya.”
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Whatever (Late) Shri M.K. Gandhi visualised about Rama Rajya, it is only narrating the one half of what Sage Valmiki said.
Sage Valmiki never said that it was only in the rule of Shri Rama that people lived happily and prospered well.
Sage Valmiki even said the same thing about the rule of King Dasaratha and King Janaka also.
In fact, an ideal society is to be called SATYA, DHARMA RAJYA, where not only the King or ruler will follow the path of TRUTH and DHARMA, but even the people should follow the path of TRUTH and DHARMA.
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While narrating the rule of King Dasaratha, Sage Valmiki says says as follows in the Sarga 6 of Bala Kanda of Ramayana.
तेन सत्याभिसन्धेन त्रिवर्गमनुतिष्ठता ।
पालिता सा पुरी श्रेष्ठा इन्द्रेणेवामरावती ।।1.6.5।।
King Dasaratha, who always kept his word and followed dharma, attained prosperity and pleasure in equal measure by righteous means ruled that best of the cities as Indra ruled Amaravati.
तस्मिन्पुरवरे हृष्टा धर्मात्मानो बहुश्रुता: ।
नरास्तुष्टा धनैस्स्वैस्स्वैरलुब्धास्सत्यवादिन: ।।1.6.6।।
The people living in that excellent city were happy, virtuous, well read (in the Vedas and the sastras) and content with the possessions of their own. They were free from avarice. And always spoke the truth.
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So, a king or ruler, who is following the the path of TRUTH and DHARMA, should make his people also follow the path of TRUTH and DHARMA.