For more information regarding Bristol-Myers Squibb Clinical Trial participation, please visit www.BMSStudyConnect.com Inclusion Criteria: - Localized invasive breast ductal carcinoma, confirmed by the local pathologist, that includes either T1c-T2 (tumor size ≥ 2 cm), clinical node stage (cN)1-cN2, or T3-T4, cN0-cN2. Nodal status must be cytologically confirmed. Note: Inflammatory breast cancer is allowed - ER+ breast cancer and with or without progesterone receptor (PgR) expression (determined on the most recently analyzed tissue sample and tested by the central laboratory, as defined in the relevant American Society of Clinical Oncology [ASCO]- College of American Pathologists[CAP] Guidelines - Must agree to provide primary breast tumor tissue at baseline and at surgery - Must be deemed eligible for surgery and must agree to undergo surgery after completion of neoadjuvant therapy - Males and females must agree to follow specific methods of contraception, if applicable, while participating in the trial - Must have an Eastern Cooperative Oncology Group (ECOG) scale performance status of 0 or 1 Exclusion Criteria: - 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 7 months for participants who do not receive cyclophosphamide, after the last dose of study treatment - Prior treatment with chemotherapy, endocrine therapy (ET), targeted therapy, and/or radiation therapy for the currently diagnosed breast cancer prior to enrollment - 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 - 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 protocol-defined inclusion/exclusion criteria apply