Apart from the kingdom of Lanka ruled by Ravan there were lots of Kingdoms in the then world. Some of the important kingdoms were Janakpuri ruled by King Janak, and Ayodhya ruled by King Dasrath. And certainly there were forests all around where saints lived along with Vaanars, the evolved humans. Of course forests were full of fearful animals.
None of the account of History suggests that Ravan was in any serious conflict with any of the Kingdoms. In fact after Parashuram handed over Shiv Dhanush to Janak, the weapon of mass destruction, Ravan always felt that Lanka was vulnerable. Lanka was far away from other kingdoms and was sea locked. It was an ideal target for weapon of mass destruction. As such Ravan was only causing atrocities on Vaanars and plundering forest wealth including mineral wealth from within the forest. He was plundering forest wealth and trading Vaanars as slaves and animals.
When Sage Vishvamitra took upon himself the YAGNA(Concentrated efforts) to end the atrocities of Ravan, he was aware of these problems. He knew most of the kingdoms will not help in this, as kingdoms, because of economic and other ties will be extremely reluctant to oppose Ravan.
Because of its economic power, and strong propaganda, Lanka looked formidable indeed. But Ravan was NOT INVINCIBLE. And this has been proved by History. When Ravan kidnapped Sita, the ruling queen of Ayodhya, Ram had the option of calling for the forces of Ayodhya, Janakpuri, other friendly kingdoms. But Ram preferred to use Vaanars. Though Vaanars were also trained to use modern weapons, they, nevertheless looked less powerful than the sophisticated army of kingdoms. Yet they defeated Lankan forces quite comprehensively.
Ramayan as history CLEARLY tells us that the main reason for Lord Vishnu’s avatar as Shri RAM was to ensure:
- Rehabilitation of Vaanars as Human
- Declare Agni Pariksha as an ADHARM
- Establish Ram Rajya.