Sidney begins this sonnet with four lines in which he praises himself for his "cunning in warlike sports," in this case, battling his opponents with wooden staffs. The response from the crowd is so positive, he says, that his "youth, luck and praise" made him want to keep on with this mock-fighting.
The turn in the sonnet comes in the next four lines when Cupid sees the poet, who acknowledges that he's Cupid's slave but "prancing" around in the garments of war and ready for more war games. Cupid's response is almost jealous when he says I would like you to pay the same attention to love that you pay to warlike pursuits. And then Cupid points up and says, "Look here, I say"
Courtly love is the focus of the next four lines when the poet sees his love, Stella (meaning, "star''), who, just like a star, sends light from the window. The poet, who is still clothed for war, is completely undone: first, he loses control of his horse; then, he forgets to defend himself. He even fails to hear the trumpet signal the beginning of a charge, and the cries of the onlookers warning him of his danger.
The concluding couplet is actually quite funny: when his opponent charges, the poet takes off and leaves the opponent "chasing me." But his lady's blush at his un-warlike behavior cause him to feel shame at his flight.
The theme of the sonnet, then, is the power of love to completely unstring the senses of the lover and force him to do something completely uncharacteristic--in this case, unceremoniously leave the field of battle.
This sonnet, as with all of the Astrophil and Stella sequence by Sir Philip Sidney, deals with the impact of love on the speaker as he seeks to articulate the change that has come upon him now that he has fallen in love with Stella and been inspired. The poem imagines the speaker showing his valour in battle in a kind of gladiatorial competition. Again and again he is victorious in these "martial sports," and he is proud and invigorated by his achievements:
While, with the people's shouts, I must confess,
Youth, luck, and praise, ev'n fill'd my veins with pride...
However, it is at this stage that Cupid, as the god of love, appears at the arena and recognises that the speaker, for all his apparent devotion to Mars, the god of war, is actually a servant of love. The poem then describes how the speaker caught sight of the fair Stella, and as a result completely lost his composure and focus on battle:
My heart then quak'd, then dazzled were mine eyes;
One hand forgot to rule, th'other to fight.
As a result of this, the speaker completely ignores his next opponent and is crushed, showing that he is completely overpowered by love and is in the power of Cupid rather than Mars. The final line tells us that it is only the "blush" of Stella that indicates to the speaker his "shame" and being so completely unmanned by love in its totality.