This phrase, "all who were able to go to war," appears 14 times in this one section! What is the significance of God commanding a census to be taken of all males twenty years old and above and able to go to war? Obviously, God wanted Israel to comprehend the size of its army. God was preparing the Israelites for the certainty of warfare with the people of the land.
Notice the precise dating in verse 1: It was "the first day of the second month, in the second year" following their flight from Egypt. Exodus 19:1 tells us, "In the third month after the children of Israel had gone out of the land of Egypt, on the same day, they came to the Wilderness of Sinai." The giving of the law, then, occurred three months after they left Egypt. Exodus 40:17 adds another time marker: "And it came to pass in the first month of the second year, on the first day of the month, that the tabernacle was raised up."
Thus, after receiving the law and making the covenant, God gave instructions concerning the Tabernacle, and it was constructed. It was erected on Abib/Nisan 1, the first day of the first month, of the second year. At this point, the Israelites have now been out of Egypt just two weeks shy of a full year. Once the Tabernacle had been erected and was in operation, God gave to Moses all the instructions in the book of Leviticus, and the priesthood began to put the ceremonial practices into operation.
In Numbers 1:1, thirteen more months have passed, and Israel is still at Mount Sinai. However, their stay is soon to end. The events of Numbers 13 and 14 did not take place until near the end of this second year after leaving Egypt. Therefore, before the Israelites even began the serious part of their trek to take over the Promised Land, the initial preparation for their being forged into an army, a force intended to fight against the people of the land, had already taken place.
They fully understood that Exodus 23 was not an unconditional promise that God would fight all their wars for them. They knew they would not just walk into the land and take it over while hornets maddeningly buzzed around their enemies' heads!
John W. Ritenbaugh
The Christian Fight (Part One)