Complex sentence: Around 300 BC, Gojoseon lost significant western territory after a war with the Yan state, but this indicates Gojoseon was already a large enough state that could wage war against Yan and survive the loss of 2000 li (800 kilometers) of territory.
Split into simpler sentences: Around 300 BC, the kingdom lost significant western territory after a war with the Yan state. This could indicate that a Gojoseon was a large enough state that could wage war against Yan and survive the loss of 2000 li (800 kilometers) of territory.