加盟[kamei] participation
加盟国[kameikoku] member-nation
加盟者[kameisha] participants
加盟店[kameiten] participating store
軍事同盟[gunjidoumei] military alliance

三国同盟[sangokudoumei] triple alliance
盟う[chikau] to swear
同盟休校[doumeikyuukou] student strike
同盟軍[doumeigun] allied armies
同盟国[doumeikoku] ally (of another nation)