War against another nation
Micronations can declare war on other micronations just like other countries.
Prosecuting the war
Micronations usually fight war jokingly, since if the two micronations would actually fight with weapons, the actual recognized countries nearby might declare the micronations violent (although in some cases, such as during the Molossian-Mustachistani War, surrounding recognized countries instead remained neutral), and try to take over.
Ending the war
Micronations most commonly end wars with peace treaties, declaring peace between the two nations.