For more information regarding Bristol-Myers Squibb Clinical Trial participation, please visit www.BMSStudyConnect.com. Inclusion Criteria: - Participant has untreated, unilateral invasive, histologically confirmed, localized invasive breast ductal carcinoma that includes either T1c-T2 (tumor size ≥2 cm), clinical node stage (cN)1-cN2, or T3-T4, cN0-cN2. Note: Inflammatory breast cancer is allowed. - Participant has centrally confirmed ER+ invasive breast cancer with or without progesterone receptor expression, according to the most recent American Society of Clinical Oncology/College of American Pathologist guidelines. - Participant must agree to provide tumor tissue at baseline and at surgery. - Participant must be deemed eligible for neoadjuvant chemotherapy. - Participant must be deemed eligible for surgery and must agree to undergo surgery after completion of neoadjuvant therapy. - Women and men must agree to follow instructions for methods of contraception. - Participant must have an Eastern Cooperative Oncology Group (ECOG) scale performance status of 0 or 1. Exclusion Criteria: - Participant who is breastfeeding, pregnant, or expecting to conceive or father children within the projected duration of the study, starting with the screening through 12 months for participants who receive cyclophosphamide, or 6 months for participants who do not receive cyclophosphamide, after the last dose of study treatment. - Prior treatment with chemotherapy, endocrine therapy, targeted therapy, and/or radiation administered for the currently diagnosed breast cancer, or where upfront chemotherapy is judged clinically inappropriate as optimal neoadjuvant treatment. - Prior treatment with an anti-PD-1, anti-PD-L1, anti-PD-L2, or anti-CTLA-4 antibody, or any other antibody or drug specifically targeting T-cell co-stimulation or checkpoint pathways, or history of allergy, or hypersensitivity to study medication. - Participant has significant cardiovascular disease such as left ventricular ejection fraction (LVEF) < 50% at baseline as assessed by echocardiography (ECHO) or multigated acquisition (MUGA) scan performed at screening, or Class III or IV myocardial disease as described by the New York Heart Association. Other inclusion/exclusion criteria may apply.