This is the part when he is on horseback, about to warn the town that the British are coming. In the first line, he starts heading toward the village streets with "the fate of a nation" counted on him to do so. This because he is about to warn everyone so that they can be prepared to fight. This is shown through the lines "the spark struck out by that steed", meaning that the villagers are counting on his warning to change history by sparking the Revolution.
