Answer:
The missing words to complete the sentence "______________ broke away from Russia during the Russian Civil War and were able to maintain their independence until the early years of World War II" are "Estonia, Latvia, and Lithuania".
This information is provided by search results [1], [2], and [3], which all mention these three Baltic States as having declared independence from Russia during the period of the Russian Civil War (1917-1923) and being able to maintain their independence until World War II broke out.
Step-by-step explanation: