According to the employees, Google promoted a culture of employees challenging and debating product decisions - but this has been eroded.
Project Maven helps process aerial drone footage using artificial intelligence (AI) to spy on vehicles and even follow people as they come and go in and out of buildings.
Google's involvement sparked ethical concern and anger among employees, Gizmodo initially reported.
Exact reasons for leaving vary from the obvious ethical issues, both of drone warfare and the implementation of AI as part of it, through to the idea that Google's involvement is discrediting the company and that those people don't wish to be associated. "And if I can no longer recommend other people to this company, why am I still here?" - quoted by Gizmodo, one of the dismissed employees. It is thought to be one of the lead contenders for another significant Pentagon cloud computing contract, the Joint Enterprise Defense Infrastructure, better known as JEDI, that is now up for bids. When the story first broke, it told Gizmodo: "The technology flags images for human review and is for non-offensive uses only".
"Any military use of machine learning naturally raises valid concerns".
Google has defended their work and in a statement made last month, the company said, "The technology is used to flag images for human review and is meant to save lives and save people from having to do highly tedious work".
"We're actively engaged across the company in a comprehensive discussion of this important topic and also with outside experts, as we continue to develop our policies around the development and use of our machine learning technologies".
However members of the tech industry are anxious. "Google is supplying not only the open source "deep learning" technology, but also engineering expertise and assistance to the Department of Defence".
"The DoD contracts under consideration by Google, and similar contracts already in place at Microsoft and Amazon, signal a unsafe alliance between the private tech industry, now in possession of vast quantities of sensitive personal data collected from people across the globe, and one country's military", the letter reports. The contract, for those wondering, is for a project known as Project Maven.
ZDNet has asked Google for comment and will update the story if it receives one.
Project Maven was commenced in April of 2017 by then-Deputy Defense Secretary Bob Work, who started an Algorithmic Warfare Cross-Functional Team.
Project Maven is essentially a research initiative to develop computer vision algorithms that can analyze drone footage.