True, a new mistress now I chase, The first foe in the field;
Or maybe he is cheating. He refers to his new mistress—one he's now chasing, instead of, presumably, Lucasta.
And that mistress? It's his enemies in war—"the first foe in the field."
Okay, what's up with the metaphor here? How in the world could an enemy, who's trying to kill him, be anything like a mistress?
Well, we think this suggests that for our speaker, there's something alluring about war and battle. Something irresistible.
What, we're not sure. We guess we'll have to keep on reading to see if the speaker will give up the goods.
And with a stronger faith embrace A sword, a horse, a shield.
More on the mistress. Frankly, it sounds like our speaker is cheating on his poor Lucasta with none other than warfare itself.
In fact, it sounds like he likes war—the sword, the horse, the shield—more than he likes Lucasta. After all, he hugs them with "stronger faith." Yikes. These two need to get into couple's counseling, ASAP.