I got the impression that it was during the Great War when assembly lines probably ran 24/7/365 to throw as much as possible at the Covenant. Most designs from the MA5 to frigates existed long before the Covenant invaded Harvest, so there was no screwing around on a massive scale with unproven kit. Only looking at the dozens of intricate post-war MJOLNIR armor lets me know that it's all gone to shit on a level not seen since the years following the Second World War, or now, in the post-Cold War era.