How do you harmonize the two accounts of David's census in 2 Samuel 24 and 1 Chronicles 21?

First, why was it wrong for David to number the army at all? Because Israel's confidence was to rest on trust in God, not on numbers; if God says He will be with you, it does not matter whether you have one million or five. God had proved this again and again, as with Gideon's three hundred men, but David had a lapse of faith and wanted to be sure he had enough. Now to the discrepancies. 2 Samuel 24:1 says the Lord incited David against Israel, while 1 Chronicles 21 says Satan stood up and incited David. One explanation is that Satan does nothing except what God allows, so God used Satan to incite David; the devil thought of it, and God permitted it, intending to punish David for acting by his own free will. Another explanation is grammatical: in 2 Samuel 24 the Hebrew has no explicit subject for 'incited,' so we only know the Lord's anger was kindled, while 1 Chronicles 21 supplies Satan as the subject. Comparing Scripture with Scripture, we conclude Satan did it with the Lord behind it. As for the numbers, 2 Samuel says the count took nine months and twenty days and gives 800,000 in Israel and 500,000 in Judah, while 1 Chronicles gives 1,100,000 and 470,000; 1 Chronicles does not state how long the count took and says 'all Israel' twice, suggesting a longer or second phase, and it notes that Levi and Benjamin were not included, which accounts for the difference. As for the famine, both texts actually say three years, so there is no discrepancy there.