expression, godan verb (-bu)
form an alliance; enter into a pact
A somewhat formal expression for making a formal agreement or alliance, often between groups or nations.
両国は平和のために盟を結んだ。
The two countries formed an alliance for peace.
同盟を結ぶ is more common and neutral for 'form an alliance'; 盟を結ぶ is slightly more literary and formal.
The expression combines 盟 (alliance, oath) with 結ぶ (to tie, to form). The exact historical derivation is uncertain, but it follows a common pattern of using 結ぶ with abstract nouns to mean 'to form' or 'to enter into'.