The red cross sign has a very specific meaning and protection under international law. They don't want the symbol to be used outside of the agreed uses because they don't want that meaning, and consequently the protection it affords, to get muddled.
They also don't like when a red cross is used on a random first aid kit in the real world.
Everyone is betting Tonga will not get itself into a war.
But if it hypothetically happened, I assume they would be pressured to use an alternative war flag by the international community.
On a more serious note, applying international laws against sovereign states is always more complicated than to enforce them against individuals and organizations operating inside of signatory states.
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