Mark Twain was in Whanganui December 3-5 and December 7-9, 1895. On December 8, a Sunday, he took a river boat excursion that included a visit to Moutoa Gardens and saw a monument dedicated to those killed in the Battle of Moutoa Island, which took place in 1864. This visit prompted him to write a rather powerful and poignant statement about patriotism. He was misinformed or at least mis-remembered the visit. The monument morphed into two separate monuments, in his mind. The first monument was one "in honor of white men 'who fell in defense of law and order against fanaticism and barbarism.'". The second "to Maoris who fell fighting with the whites against their own people." There is only a single monument and there were no white combatants involved.

"At Moutoa, cousin fought cousin. In May 1864, Matene Te Rangitauira led 300 anti-European Pai Mārire supporters (Hauhau) from the upper Whanganui River in an attack on Whanganui town. Their path was blocked at Moutoa, a small island in the river near Rānana (London). There, on 14 May, they faced kūpapa led by two chiefs from Pūtiki, near the rivermouth, Hōri Kīngi Te Ānaua and Hoani Wiremu Hīpango."

"Although it lasted only half an hour, the Moutoa engagement cost the lives of about 50 Hauhau and 15 kūpapa. A European, lay brother Euloge from a nearby Catholic mission, was fatally shot as he stood on the riverbank."
https://nzhistory.govt.nz/media/photo/moutoa-gardens-nz-wars-memorial

Robert Cooper, in "Around the World with Mark Twain" (pp164-5) explains:
"Both parties to the conflict were Maori. The defenders of law and order were members of the Lower Wanganui tribe, who had established profitable trading links with the port of Wanganui, and members of other downriver tribes. The representatives of fanaticism and barbarism were members of a religious movement, Pai Maire (meaning 'good and peaceful'), which arose from Maori bitterness over land confiscations."

"Adherents believed that shouting "Pai Mairie, Hau! Hau!" in battle would shield them from the white man's bullets, ..."

"In that year Hauhaus from the Taranaki and Upper Wanganui tribes determined to attack Wanganui township and asked the Lower Wanganui for support. The latter, reluctant to abandon their profitable trade with the settlement, refused. "

"Hauhaus and the downriver tribes met at Moutoa Island, in the middle of the Wanganui River. Both sides faced each other in the middle of a long field, like two football teams, while spectators lined both sides of the river to watch. After a brief and bloody battle, the downriver tribes repulsed the invaders. Forty corpses were left on the field, including Brother Euloge from the nearby Catholic mission. According to one account, he was shot dead while begging the combatants to stop fighting. His name and those of fifteen dead warriors from the downriver tribes appear on the monument."