Dennis already said stuff about it but he's Dutch and honestly, it's Dut really even a country?

But yeah because I am a halofag I know the lore. Canonically, the Humans actually won ground engagements a surprising amount of the time even without Spartans. The problem was never, really, the ground engagements, which were messy and costly but where we could do well enough to win. In fact we had been so damned tenacious that the Elites, especially, were starting to ask "Hey why haven't you made peace and incorporated them into the Covenant yet?" because they reckoned we'd earned it, and Truth's little crusade had always been a little fishy. It was in space where we lost the war, because Covie ships both outnumber and outclass Earth's on a huge scale (The Pillar of Autumn being an exception because Keyes is a smurfing badass and Cortana is also rather neat at being in control). The Autumn has a crew of around 2500, of which not even close to all survive down to Halo, but the humans still provide severe resistence to the numerically far-greater Covenant, and would have done so even without the Chief there.