Rita C., a Native American member of the Tohono O’odham Nation tribe (Tribe) (defendant), gave birth to Baby Boy C. in March 2004. The father, Justin W., was not Native American. Rita grew up in a tribal community but eventually left it and no longer participated in its political, social, and religious affairs. Her other children were not being raised in a tribal community. In April 2004, Rita and Justin entered consents in an Arizona court to the child’s adoption by a non–Native American couple, Jeffrey A. and Joshua A. (plaintiffs), who lived in New York. Rita’s consent noted her heritage and the possibility that the child was an “Indian child” pursuant to the Indian Child Welfare Act (ICWA), which established certain placement preferences for Indian children no longer in their parents’ custody. The Tribe was notified of the proceeding but did not appear. The court accepted Rita’s waiver of preferences for the baby’s placement and her request that good cause be found to place the baby with Jeffrey A. and Joshua A. The prospective adoptive parents petitioned to adopt the child in a New York court. The Tribe intervened. The trial court concluded that the ICWA was inapplicable because the Tribe did not prove that the child was part of an “existing Indian family.” It therefore allowed the adoption to proceed. The Tribe appealed.