British troops were out moving across the back bay. In Concord and Lexington, trained militiamen were waiting to respond. Where and approximately when is this?

Check this site for your answer. Note: the Back Bay is in Boston.

http://www.kidport.com/REFLIB/UsaHistory/AmericanRevolution/AmerRevolution.htm

Based on the provided information, the events described, with British troops moving across the Back Bay and militiamen waiting to respond in Concord and Lexington, are part of the American Revolutionary War. To find out the specific location and approximate time of these events, you can visit the website mentioned (http://www.kidport.com/REFLIB/UsaHistory/AmericanRevolution/AmerRevolution.htm) and look for relevant information related to the American Revolution.