Moses recalls the writing of the second tablets, the ark he was commanded to make for it, which was different from the ark of the Tabernacle. It was the former that was taken out to war, except in the days of Eli, when the latter was taken out during a war with the Philistines. This was a big mistake and the Philistines captured the ark -- a lesson in making sure religion doesn't become detached from G-d.
