How to say "annexation" in Japanese

• 付加ふかCommon
  suru verb, transitive verb, no-adjectiveIts core senses are addition and appendage, so it fits annexation only when this means appending something to what already exists; for the annexation of territory it is seldom the word you want.
  スミスのアプローチは、接辞せつじ付加ふかということを考慮こうりょに入いれない点てんでほかと非常ひじょうに異ことなっている。↗

  Smith's approach differs greatly from others in that he did not take the affixation into consideration.
• 合併がっぺいJLPT N1approx.Common
  noun, suru verb, transitive verb, intransitive verbAnnexation in the sense of a merger: companies, towns and the like being combined into one. This is the word for corporate mergers and for the amalgamation of towns and municipalities.
  ２社しゃは合併がっぺいを計画けいかくしている。↗

  The two companies plan to unite.
• 併合へいごうCommon
  noun, suru verb, transitive verb, intransitive verbAnnexation as two things joining into one; the nuance is of separate bodies being melded into a single whole. It is used for a state or territory being annexed by another.
  その帝国ていこくはすべての小国しょうこくを併合へいごうした。↗

  The empire absorbed all the small states.

Use 合併 for the merger of companies or towns, 併合 for territories or states joining into one, and 付加 only when annexation means adding or appending something.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 付加ふかaddition
• 合併がっぺいmerger (of companies, towns, etc.)
• 併合へいごうmerger
• 併呑へいどんannexation
• 別添べってんattachment