The so called conquest of Canaan by the Israelites described in the book of Joshua is pious fiction. During this time dated by biblical chronologists (14th-13th century BCE), Canaan was a province of Egypt. Egyptian soldiers were all over Canaan defending the area from any foreign invasions. Cities that the book of Joshua described were not occupied during that period and some where destroyed centuries before.
Jerusalem during the time of David and Solomon was a rural village. their so-called United Empire never existed. Canaan was ruled by two kingdoms. the Stronger, richer cosmopolitan Kingdom of Israel to the North and the rural, poorer Kingdom of Judah to the South. After the Assyrian destruction of Israel. Judah under the rule of Josiah invented the pious fictional story of the united Kingdom under David and his son Solomon the wise king. Josiah wanted to add the lands in north previously ruled by Israel and united it and Judah to form a new united Israelite Kingdom with Jerusalem as the capital. it is the Kingdom of Judah that wrote the entire story with a Judah/Jerusalem bias.
Part 4 coming next week