We prepare hard for the high school entrance examination and win the title of British Prime Minister to show our ambitions.

It is a metaphor for making preparations in advance and showing off your skills to achieve it. "Strong troops and horses" means to sharpen weapons and feed war horses, describing being ready for war, and also a metaphor for making preparations beforehand. Winning the British Army and displaying one's ambitions means to implement a grand and ambitious plan or ambition on a large scale, which is a metaphor for realizing the goal. The two together mean to make preparations in advance and to show off your skills to achieve them.