At once upon the outbreak of hostilities at Lexington, the conquest of Canada had been contemplated by the Northern leaders, who well remembered how, in days gone by, the valley of the St. Lawrence had furnished a base for attacks upon the province of New York, which was then the strategic centre of the American world. It was deemed an act of military prudence to secure this region at the outset. But so long as the least hope of conciliation remained, Congress was unwilling to adopt any measures save such as were purely defensive in character. As we have seen, it was only with reluctance that it had sanctioned the garrisoning of Ticonderoga by the Connecticut troops. But in the course of the summer it was learned that the governor of Canada, Sir Guy Carleton, was about to take steps to recover Ticonderoga; and it was credibly reported that intrigues were going on with the Iroquois tribes, to induce them to harry the New England frontier and the pleasant farms on the Hudson: so that, under these circumstances, the invasion of Canada was now authorized by Congress as a measure of self-defence. An expedition down Lake Champlain, against Montreal, was at once set on foot. As Schuyler, the commander of the northern department, was disabled by ill health, the enterprise was confided to Richard Montgomery, an officer who had served with distinction under Wolfe. Late in August, Montgomery started from Ticonderoga, and on the 12th of September, with a force of two thousand men, he laid siege to the fortress of St. John's, which commanded the approach to Montreal. Carleton, whose utmost exertions could bring together only some nine hundred men, made heroic but fruitless efforts to stop his progress. After a siege of fifty days, St. John's surrendered on the 3d of November, and on the 12th Montgomery entered Montreal in triumph. The people of Canada had thus far seemed favourably disposed toward the American invaders, and Montgomery issued a proclamation urging them to lose no time in choosing delegates to attend the Continental Congress. [Sidenote: Arnold's march through the wilderness of Maine]