A force of Titan armored transports under heavy escort. Each transport carries a company of colonial marines and the entire fleet is guarded by a battleship, three cruisers, two escort carriers and a half-dozen frigates.
To take a world through force of arms is a much larger endeavor than most people think. In most situations, taking a planet by force is considered an option of last resort. Most planets are huge, with multiple continents and terrains, potentially millions or billions of inhabitants, a 360 degree spherical approach for enemy reinforcements and supply drops, and countless square miles to hide insurgents, listening posts, even entire military bases.
In most cases, the capture of strategic goals is enough to start, including starports, interstellar communication hubs, and capitol cities. Traffic to and from the planet must be halted, and the major mass of the enemy on the planet must be destroyed. After that, the planet can be used for strategic goals, even if it has not been completely pacified.
Then there's the risk of getting a landing force to an enemy-held planet safely. Interception by the enemy can be disastrous.
And then, once you've secured a beachhead on the planet, you need to secure the supply lines, make sure you maintain space dominance and if the fortunes of war turn suddenly, you may have to evacuate hundreds of thousands of troops from across an entire planet with only hours notice.
All while watching out for an enemy that wants to take advantage of any error to derail your plans.
Very few interstellar governments have the ability to take a planet. The Terran Protectorate is one of them.