A Study To Evaluate the Efficacy and Safety Of Atezolizumab or Placebo in Combination With Neoadjuvant Doxorubicin + Cyclophosphamide Followed By Paclitaxel + Trastuzumab + Pertuzumab In Early Her2-Positive Breast Cancer

Inclusion Criteria:

* Confirmed diagnosis of HER2-positive breast cancer, and hormonal and PD-L1 status, as documented through central testing of a representative tumor tissue specimen
* Primary breast tumor size of \> 2 cm by any radiographic measurement
* Stage at presentation: T2-T4, N1-N3, M0 as determined by AJCC staging system, 8th edition
* Pathologic confirmation of nodal involvement with malignancy must be determined by fine needle aspiration or core-needle biopsy. Surgical excision of lymph nodes is not permitted.
* Patients with multifocal tumors are eligible provided at least one focus is sampled and centrally confirmed as HER2-positive.
* Patients with multicentric tumors are eligible provided all discrete lesions are sampled and centrally confirmed as HER2-positive.
* Eastern Cooperative Oncology Group Performance Status of 0 or 1
* Baseline LVEF \>= 55% measured by echocardiogram (ECHO) or multiple-gated acquisition (MUGA) scans
* Adequate hematologic and end-organ function obtained within 14 days prior to initiation of study treatment
* For women of childbearing potential: agreement to remain abstinent or use contraceptive methods, and agreement to refrain from donating eggs
* For men: agreement to remain abstinent or use contraceptive measures, and agreement to refrain from donating sperm

Exclusion Criteria:

* Prior history of invasive breast cancer
* Stage IV (metastatic) breast cancer
* Patients with synchronous bilateral invasive breast cancer
* Prior systemic therapy for treatment of breast cancer
* Previous therapy with anthracyclines or taxanes for any malignancy
* Ulcerating or inflammatory breast cancer
* Undergone incisional and/or excisional biopsy of primary tumor and/or axillary lymph nodes
* Sentinel lymph node procedure or axillary lymph node dissection prior to initiation of neoadjuvant therapy
* History of other malignancy within 5 years prior to screening, with the exception of those patients who have a negligible risk of metastasis or death
* Cardiopulmonary dysfunction
* Dyspnea at rest
* Active or history of autoimmune disease or immune deficiency
* Pregnancy or breastfeeding, or intention of becoming pregnant during study treatment or within 5 months after the final dose of atezolizumab/placebo, 6 months after the final dose of doxorubicin, 12 months after the final dose of cyclophosphamide, 6 months after the final dose of paclitaxel, or 7 months after the final dose of trastuzumab, pertuzumab, or trastuzumab emtansine whichever occurs last

Exclusion Criteria Related to Trastuzumab Emtansine in the Adjuvant Setting:

* Patients who achieved pCR
* Evidence of clinically evident gross residual or recurrent disease following neoadjuvant therapy and surgery
* Unable to complete surgery with curative intent after conclusion of neoadjuvant systemic therapy
* Patient discontinued treatment with trastuzumab because of toxicity during the neoadjuvant phase of the study
* Clinically significant history of liver disease, including cirrhosis, current alcohol abuse, autoimmune hepatic disorders, or sclerosis cholangitis
* Patients with Grade \>=2 peripheral neuropathy
* Prior treatment with trastuzumab emtansine