In episode 7 of Chief of War, "The Day of Spilt Brains", Captain Simon Metcalfe commits a horrific massacre after being rejected on trade by Kamehameha.
While leaving the Island, he poses an attack on a nearby village by the bay. Loading his canons, he attacks innocent women and children of the village, making Kamehameha realize the consequences of his decision. Ka'iana is unable to reach the village on time and is burdened with guilt and grief of losing his people.
Earlier in the episode, Ka'iana tries to make Kamehameha see through these white men, as they don't consider Hawaiian people as human but only savages. Kamehameha doesn't take it seriously and keeps up with his law of choosing peace and diplomacy over violence. This results in Metcalfe actually showing his true colours, but at the cost of several villagers' lives.
Since Metcalfe and Marley arrived, Ka'iana feared the looming threat of colonizers and considered it an even greater threat than the internal rivalries. What will be the next step for the islanders? Will the Island unite against colonizers to protect its land?
Is there a chance of the Hawaiian kingdom uniting to fight Captain Metcalfe in the Chief of War?
According to what the Hawaiian history already suggests, the Metcalfe atrocity would work as a catalyst to finally bring together the united chiefs. Along with this, Ka'iana's militant approach has gained more credibility, making him a good military leader of the collective fight against colonizers as Chief of War.
In episode 6, we see Keoua joining hands with Kahekili to wage a war on Kamehameha. Kahekili, though he agrees to support Keuoa, is planning to kill him as he becomes useless to him when he has control over the island himself. Kamehameha still leans toward rules and peace; Ka‘iana leans toward war. Their conflicting philosophies create friction.
With episode 7, it seems like the show is building up on the colonial tension. It also reframes the entire conflict as it isn't about each one's island but a shared, merciless enemy. Kamehameha and Ka'iana have understood this first-hand, but others are still under the influence of more power and greed. Therefore, having said that, the path to unity won't be smooth or sudden.
How will Kamehameha and Ka'iana unite the island against a more cruel enemy, in Chief of War?

According to history, Metcalfe's attack did lead to the unification of the island. However, the events differed a bit from the show's interpretation. When Captain Simon Metcalfe of the Eleanora first arrived in Hawaiʻi, he was greeted by Chief Kameʻeiamoku.
However, their relationship gets strained when Metcalfe doesn't abide by some of his rules. This angers the King while Matcalfe sails to another island, Maui. There he lands up in bigger trouble with one of his sailors going missing. Metcalfe seeks revenge. At Olowalu, he tricks villagers by pretending to trade, then fires his cannons at close range into their canoes. Around 100 Hawaiians were killed, and many more were wounded.
Surely some political manoeuvring and alliances need to be formed. Ka'iana will have to strengthen bonds with the neighbouring islands, and Kamehameha will need to adjust strategies, connecting with other chiefs. The prophecy is something that unites the islands in Chief of War; therefore, Taula could also play a major role in some cultural or spiritual awakening of the chiefs.
The image of their Hawaiian village with several people, including Vai, dead, might also provoke a sense of unified retaliation against a greater force in the Chief of War.