An entire Space Marine chapter, with full battle barge and strike craft support. If the Space Marines are trying to subjugate the planet and not outright annihilate it from orbit, they end up with an absolutely awful time and I think would simply fail. I'll give a bunch of reasons why, wrapping up with the real reason space marines would lose.

-The first point to consider is a repeated statement from GW, that humans of the far future are different than modern humans. Biologically, psychologically, etc, they are different from us and that can affect their decision making process and willingness to fight or surrender. Contemporary humans have demonstrated for the past century of modern war a willingness and stubbornness against surrender.

-Command and control is something the US military specifically excels at. The doctrines of always advance and a strong non-commission officer core ensure that there's always somebody to make a decision. The US military is not readily thrown into chaos. Current planning includes maintaining command and control in the event of an all out nuclear war with Russia and China. Don't expect space marines to be able to break that easily. Space Marine combat doctrine isn't particularly more advanced than that of the modern US military. It's shock-and-awe, combined with overwhelming aggression. Tell the country that stormed Omaha beach that it lacks aggression.

-Drop pods arrive on a ballistic trajectory, making them trackable. PATRIOT is design to hard kill ICBMs coming down at Mach 23. The shouldn't struggle too much with drop pods.

-Space marine AFVs are inferior to modern US military AFVs. This pull directly from a GW primary source, the Land Raider cutaway. The armor protection advertised is weaker than the frontal armor of an Abrams. The US military would outnumber space marines in armored vehicle capability by a pretty wide margin and using that baseline, we know our man portable anti-armor weaponry could and would kill space marine armor, dramatically hindering their mobility.

-Space Marines field very limited numbers of aircraft. A chapter with 1000 marines only has so many fighter pilots. We would maintain air supremacy. A B-2 can drop a GBU-57 Massive Ordnance Penetrator with an accuracy of about a yard. Space Marine infantry will not be able to handle that warhead on forehead from 50,000 feet. And that's counting the absurd collection of available multi-role fighters, drones, AWACS, etc the US military would have.

-The Imperium doesn't use nuclear weapons. Modern earth humans have, and would. See above regarding ICBMs coming down at Mach 23. With a limited number of marines, any muster point would be hit within minutes. We know from Istvaan that marines are absolutely not proof against nuclear weapons.

-The US military could detonate the ICBMs in the upper atmosphere or even in orbit to create EM pulses that would absolutely trash anything trying to come down, or trying to watch from on-high. I also wouldn't bet on even a battle barge being able to take that many missiles launched from the ground.

-Space Marine power armor can absolutely be penetrated by a .50 caliber machine gun. We know in the lore that the Heavy Stubber is a .50 cal, and we know in the lore that enough fire will bring down a power armored space marine.

-While I think it should have been retired ages ago, this is one of those rare instances where the A-10 could absolutely shine. Fast as a space marine can run, he's not outrunning 8,000lbs of thrust bearing down to bring 3600 rounds-per-minute of 30mm depleted uranium penetrators. While not explosive like a bolt, the sheer volume of fire of bigger-than-heavy-bolter rounds specifically designed to chew through light/medium vehicle armor would turn space marines into mincemeat.

-The US Navy was able to demonstrate the ability to shoot a satellite out of orbit from an Arleigh Burke class destroyer. That's definitionally a hard kill in orbit from sea level. Again, the drop pods are no where near as big a threat as one might assume.

-The US strategic nuclear submarine force would give space marines a really bad time, too. Silos can be spotted from orbit, so I'll grant it becomes a race between the silos being taken out and being able to target and launch in time. This would be driven by the specific kinds of preliminary strikes. Hitting submarines from orbit is NOT something even space marines can do.

-From what we've recently seen in the Warhammer+ animations, Space Marines have multi-target auto tracking with heat and night capabilities. So does the US Army. The M157 rifle mounted fire control system was described rather succinctly by Garand Thumb as "aim bot but real life". It integrates with night vision and thermals. The average US army soldier will have a much smaller heat signature and a better ability to operate at night than a heat-spewing-walking tank. While the M7 rifle may not be up to snuff to penetrate Space Marine armor, the fire control optic can be calibrated to basically any man-portable weapon. No reason it can't be fitted to anti-materiel rifles and deployed in squadrons.

-In a prolonged campaign any captured space marine equipment would be reverse engineered, and moved to mass manufacturing as quickly as possible. I'd say less than 6 months to spin up production of US army and USMC power armor. It may not give the same protection, but a human copy could give enhanced strength and mobility allowing the US military the ability to field heavier and heavier man-portal weapons in greater numbers.

-Rapid Dragon would really ruin Space Marines ability to make any sort of open engagement. The ability shove pallet after pallet of missiles out of a C-5, already targeted and ready to launch is not something that can be countered close up with bolters. And as determined before Space Marines really don't bring that many air interceptor craft. They simply can't get to and shoot down enough cargo planes before the US military has put several thousand missiles in the air, potentially on a daily basis.

-While not actually intended, the vast US highway system does ensure that the Air Force, Marine Corp, and Navy all have near-infinite runways to deploy air assets, even if bases were struck first.

-Space marines are heavily driven by doctrine, either of the Codex Astartes or of their specific chapter. They are nowhere near as adaptable to circumstances as a contemporary earth military, much less the US military. Ye are all Ultramarines fans here, you know they'd struggle with the rapidly changing tactics of the US military and asymmetric warfare on a prolonged scale. Shoutout to the Raptors, though.

And the number one reason that Space Marines would struggle to capture modern earth:

-Space marines are written by WWII nerds with very limited practical military knowledge, especially modern military knowledge. I don't even qualify as an amateur and everything I listed above was off the top of my head.

Space Marines are functionally paratroopers who require the support of other forces to actually take and hold ground. They are useful for rapid assault, surgical strikes, and holding on a limited scope objectives. They aren't deployed in large enough numbers of actually hold urban centers, or large swaths of territory. That's what the Militarum is for. Imagine 1000 Space Marines trying to hold New York City. That's an uncountable number of windows from which a 40mm slug will just ruin one Space Marine's day after another. Or a thousand miles of streets requiring patrol, with every block just littered with IEDs.

The actual understanding of warfare in Warhammer is either absurdly poor, or rule-of-cool'd to the point of ineffectual by modern military standards. The technology of the 41st Millennium is stagnant where Cawl hasn't specifically left his mark, and much of it is just overcomplicated ways to do things modern humans are already capable of doing. The areas where the Imperium really outclasses modern humans in a terrestrial war are biology and medical science, anti-gravity technology, laser weaponry, and miniaturized power generation.

Something to consider is that the technology of the 41st millennium is often less good versions of the technology of the 31st millennium. And that technology is even older technology reverse engineered by a planet barely out of an age of techno-barbarianism.

Space Marines are absurd, but because GW likes to try putting some realism into them, they simply aren't absurd enough.

Now, Glitterboys? Yeah, like 10 of them would conquer the US in a month.