>>22782994
By being the strategic equivalent of an undetectable tactical nuke satellite.
They pick a vulnerable target, and hit it with so much concentration and immediacy of force that they steamroll it. Any garrison force can't stand up to a Marine attack. Once they win, they leave. By the time you even know you have to dispatch a QRF, they've left. If you by chance had a force fast enough to get there close enough to do it, it's not powerful enough to meaningfully oppose them. If you have a force powerful enough to meaningfully oppose them, they can't make it there in time. That's how Space Marines win on the tactical level. Because they never deploy without Naval support since that's how they even get there, they are rarely trapped. If they are, then they run faster than your major force, and fight harder than your outriders, so they are a monstrous cunt to catch.
On the strategic level, Marines win by hitting any vulnerable spot that compromises your ability to counterattack. When they show up in their ship, which will be at least a Frigate, they will clear an area of orbit to keep it safe. Then they attack anything you can have that can take out their ship or their mobility. Then your communications. Because they hit so hard, you literally cannot have the forces to protect all these things at once with sufficient strength. If you do, they hit other important places, like supply lines, stockpiles, manufactorum, civilians, until you can't afford to keep your comms garrisoned. Then they hiot your comms, one by one, using the communicatiosn breakdown to cripple your ability to counter their deployments. Whe nyou are finally unable to meaningfully react to their attacks, they go fucking mental and instantly attack anything that is exposed, and break down the ability of your war machine to function. As soon as it is on the edge of collapse, they hit as many command points at once to utterly shatter your army. It's all still there, and all useless to you.