Came here to make an Aliens reference, rules forbid nuking it from orbit.
As the strange ship unloaded its offensive cargo nearby the planet it had been orbiting, the celestial body's native inhabitants responded to the threat appropriately. One of several A.I. made specifically for planetary defense, having detected the foreign ship and its payload, immediately informed the corresponding officer.
"Sir," it began, "I have detected a ship that failed to identify itself in the area."
"Well, what is it doing?" The officer replied.
"Its captain appears to be dropping mines in the region, sir."
"What a strange person," came the response from a perplexed officer.
And with that, Monty Python appropriately quoted, all reasonable measures had been taken. The only thing left to do was attack a random, completely uninvolved party for no apparent reason, and thus a small fleet was mobilized to fly right past the mines toward another planet and begin sending down drop-ships to deliver Imperial Guardsmen, armed with rifles completely out of place in a space-faring civilization. These troops wasted no time fixing bayonets and marching toward the nearest living beings.