Brutus:

There is a tide in the affairs of men.

Which, taken at the flood, leads on to fortune;

Omitted, all the voyage of their life

Is bound in shallows and in miseries.

On such a full sea are we now afloat,

And we must take the current when it serves,

Or lose our ventures.

Julius Caesar Act 4, scene 3, 218–224

Brutus and Cassius are discussing the final phase of their civil war with the forces of Octavian and Marcus Antonius. Cassius has been urging that they group their forces at Sardis and take advantage of the secure location to catch their breath. Brutus, however, advocates heading off the enemy at Philippi before Octavian can recruit more men. Brutus's main point is that, since "the enemy increaseth every day" and "We, at the height, are ready to decline" (lines 216–217), he and Cassius must act now while the ratio of forces is most advantageous. "There's a tide in the affairs of men," he insists; that is, power is a force that ebbs and flows in time, and one must "go with the flow." Waiting around only allows your power to pass its crest and begin to ebb; if the opportunity is "omitted" (missed), you'll find yourself stranded in miserable shallows.