In the letter, employees asked Google to no longer participate in government projects to develop Artificial Intelligence (AI) to analyze videos and improve the accuracy of the drone targeting system.
The letter also asks Google to make clear rules to prevent companies from participating in developing war technology.
Last month, Gizmodo reported that Google would assist the United States government in Project Maven, an experimental project analyzing video footage drones using AI.
"We believe, Google should not get into the business of war," wrote the letter. "Therefore, we ask Project Maven to be canceled and Google makes, publishes and enforces clear rules, stating that Google and its workers will not develop war technology."
In an official statement, a Google spokeswoman said that in cooperation with the Pentagon, Google developed a project that was not intended to be an attacking tool.
Google also called the Pentagon "an open-source object recognition software that can be used by all Google Cloud customers" based on non-confidential data.
"We know that there are many questions regarding the use of new technologies, so these discussions with employees and also outside experts have an important role to play," the spokesperson wrote.
Google is not the only technology company that works with the US military. Amazon provides image recognition technology to DOD (Department of Defense). While Microsoft offers cloud services to military agencies.