Controlled, yes, but not conquered. The British used a number of tactics to achieve that control. Take India as a good example. They took advantage of internal divisions within this huge land mass to eventually take control, but even then an army was required and it took a long, long time. It's been many years since I took my class on South Asian history, so I don't remember specific dates, but British control over India was a long process lasting a century, not just them bombing a few coastal cities and then walking to Delhi and saying, "We win!" So in taking a planet, if you had internal divisions you could exploit, say such as with Earth, then a small army might be enough. But if one assumes a unified planet such as one typically finds in Star Wars (as in most sci-fi), then you either need an army or you settle for obliterating the planet, as others have pointed out.