Of course they could have. Just fire a Halo ring
More seriously, no. It's stated somewhere that the UNSC needed to outnumber Covenant ships three to one in order to have even odds of victory, and after the fall of Reach, humanity didn't even come close to that. I'm going to be working off game canon here, since I'm not an expert on book lore, but here's my breakdown.
Reach itself is perhaps the best example: it was the single most heavily fortified planet in the UNSC. By August 16th, it was being defended by at least 60% of the UNSC's fleet, assuming Dot's comments at the start of 'Long Night of Solace' were accurate, most of the surviving Spartan-IIs and Noble Team. From first contact at Visegrad Relay to the whole world being glassed after the Pillar of Autumn's escape took a little over a month. Now admittedly, Reach is supposed to have really taken a toll on the Covenant, but they still won it, and even if we work on the assumption that '60% of the UNSC fleet' represented the absolute total percentage of the UNSC's fleets ever present over Reach (rather than in addition to its own forces, which were probably worth at least a few percentage points by themselves), that still means humanity had a whole third less available to defend Earth than they had when they had failed to defend Reach.
As for Earth? The defence that we saw throughout Halo 2 went okay, thanks to home turf, an ambush counterattack, and an extreme numerical advantage, but even then losses were bad, as within a few minutes of the opening battle two of the major orbital defence stations had been wiped out. But even with these advantages, two of the defence platforms are still destroyed, and this by a Covenant fleet which is one-fiftieth the size of the fleet which destroyed Reach. In ODST we see a much bigger fleet appear, and absolutely no signs of effective counter attack: by Halo 3, the UNSC has seemingly been reduced to operating out of abandoned surface bases, one of which falls to a single Covenant capital ship. Even the combined UNSC and Elite forces simply don't have enough strength to prevent the Covenant from opening the portal to the Ark.
Now, let's crunch some numbers. At the start of 'The Ark', it's mentioned that the Covenant outnumber the Elites three to one. Now, if we assume that everything in that battle represents the absolute total number of remaining Covenant ships, and that a Brute-led ship is only one-third as effective as an Elite ship (i.e. about as effective as a human ship), then using our initial statement that the UNSC needed to outnumber Elite-led ships three-to-one in order to have a chance of victory, at the time of the battle above the Ark, the UNSC would have needed a ship for every Brute ship and three for each Elite vessel (a total of six to four) to have a chance of winning that fight. And that's not even considering that the Covenant fleet would have been vastly larger, since it wouldn't have had a sizeable chunk of its fleet destroyed during the schism.
Next, consider that the Elite ships (i.e. a quarter of the potential Covenant hulls) were able to glass 'half a continent' within, at most, a few days, at the end of 'Floodgate'. Now even if we assume that Hood was exaggerating the scale of destruction, and that it indeed took a few days, the fact remains that the Covenant could have glassed Earth very quickly - from the first beam being fired in New Alexandria, it only took about a week to destroy Reach. The only reason they didn't do the same to Earth was because they were too busy digging out the portal to the Ark to bother.
Lastly, it's mentioned that the fleet which destroyed Reach was 50 times the size of the one which initially attacked Earth, and I seem to remember that fleet lost about a third of its strength in the battle. So on top of everything else - even on Reach, the best defended human world in the galaxy, the UNSC lost 60% of its forces to the Covenant's (at worst) 40%. Assuming that Earth could do as well as Reach with a similar rate of attrition, and even assuming that the 'largest Covenant fleet anyone's ever seen' over Delta Halo both represented all of the Covenant's and was only one ship larger than the fleet which had attacked Reach, the UNSC's navy would be completely wiped out with more than half of the Covenant's own fleet remaining. Even if we assume that High Charity only accounted for just over the mass lost at Reach (which it clearly doesn't) and the rest of the fleet was only the 2/3 which had remained after that campaign, the rate of attrition would still comfortably tip into the Covenant's favour, since even allowing for High Charity being destroyed an an unfavourable engagement on Earth they'd still end the battle of Earth with about 30% of their initial forces intact.
So to sum up: in order to defeat a unified Covenant, by the time of the battle over the portal to the Ark, humanity would have needed to outnumber the Covenant three to two in order to have a chance of winning, despite having already lost at least 60% of their forces, would have had to muster those forces about a third faster thank to the Covenant glassing a third faster, and that's on the assumption that the Covenant didn't bring the ships that would have otherwise been destroyed in the Great Schism anyway.
Edited by RedStarRocket91, 06 September 2018 - 10:51 PM.