The phrase "aggressive kingdom" is correct and usable in written English.
It can be used to describe a kingdom characterized by militaristic or hostile behavior, often in a historical or fantasy context.
Example: "The aggressive kingdom expanded its territory through relentless conquests, leaving neighboring lands in turmoil."
Alternatives: "militant realm" or "hostile empire".
Some historians believe the treasure could be loot from a battle in Mercia in central England, one of the era's most powerful and aggressive kingdoms, according to the museum.
Mercia was one of Britain's largest and most aggressive kingdoms, stretching from the Humber to London, its kings and chieftains mounting short but ferocious wars against all their neighbours, and against one another: primogeniture had to wait for the Normans, so it was rare for a king to reign unchallenged and die in his bed.
Last month's protests in Jordan were the most aggressive in the kingdom in the two years since popular revolutions erupted around the region.
Saudi Arabia intervened in Yemen, blockaded Qatar, and demanded that Lebanon's Prime Minister resign, its most aggressive actions since the kingdom was created, in 1932.
It may have been seized by one of the seventh-century Mercian kings — men like Penda, Wulfhere and Aethelred — who pursued an aggressive, plundering policy toward neighboring kingdoms.
Successive U.S. Administrations supported the Sauds and provided military assistance whenever aggressive neighbors, like Iraq, threatened the kingdom.
Veera Ballala II, the most effective of the Hoysala rulers, defeated the aggressive Pandya when they invaded the Chola kingdom and assumed the titles "Establisher of the Chola Kingdom" (Cholarajyapratishtacharya), "Emperor of the south" (Dakshina Chakravarthi) and "Hoysala emperor" (Hoysala Chakravarthi).
And in response, the kingdom is becoming more militarily aggressive, even as it struggles financially due to the global collapse in oil prices.
In return for ensuring a steady global supply of oil, successive US administrations supported the Saud family and provided military assistance whenever aggressive neighbors like Iraq have threatened the kingdom.
Veera Ballala II defeated the aggressive Pandya when they invaded the Chola kingdom.
