Why did the Guptas not conquer the whole of the Indian Peninsula

May 2013
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The abode of the lord of the north
The only part they didn't bother to conquer within the peninsula, was the vakataka kingdom. Now if you want to see why they let them be, you have to note that the expansionists of the Gupta empire were just Chandragupta, Samduragupta and Vikramaditya. Everyone else were content in statesmanship. Didn't bother to go to war unless provoked.
During the reign of expansionist Samudragupta, Vakatakas had marital alliance with guptas. Marital alliance was an important kind of geo-politics which allowed Guptas to effectively control western costs. It ensured that Vakatakas effectively remained as vassals, paying perhaps lesser tax than others, or no tax at all, not-mattering-which vakatakas never disturbed Gupta domains. It can be called effective vassalage if you want to name it so.
By the time Vakatakas finally started to revolt, expansionsm was gone from the guptas. They had more severe problems at the huns that they didn't bother to shut to down vakatakas.