The plan was in keeping with the main purpose of the war, and was, on the whole, well adapted to insure success. The northern provinces were far distant from the city of Mexico; the hold of the central government upon them was but slight; and, even if its available forces had been sufficiently strong and effective to send the troops needed to resist invasion, the difficulties of transportation would have been hard to overcome. Of course, similar difficulties were experienced in throwing the United States troops into the interior of northern Mexico; but such operations were far easier for a strong government with abundant resources than for one so ill established and so lacking in means as that of Herrera or Paredes. The population of the north Mexican provinces was sparse and unenergetic, and could not be relied on for its defence; the local governments were weak and inefficient; and in 1846 that of California was disastrously affected by dissensions between two rival leaders, José Castro and Pio Pico, representing respectively the northern district and the southern.[121] It was in the northern district, in the lower valley of the Sacramento River and near the bay of San Francisco, that the foreign population, including the Americans, was most numerous. The plan for a campaign directed at the city of Mexico was gradually developed as the war went on. The impression of Polk and his advisers at first was that a vigorous invasion of Mexico would end the war, without the necessity of pushing it far into the interior; and, since operations on the coast in the summer were so dangerous, the attack was made first in the north. The resistance of the Mexicans was, however, more desperate and prolonged than was expected, and ultimately the change was made to the shorter and more direct line of advance by way of Vera Cruz. [Illustration: MAP ILLUSTRATING THE MEXICAN WAR 1846–1848] The occupation of New Mexico and California was accomplished speedily and with little resistance. General Kearny occupied New Mexico in the summer of 1846, and the occupation of California under Commodore R. F. Stockton was completed by January, 1847. The first expeditions against Mexico from the north under Wool and Doniphan were inconclusive. The army which was most depended on to force Mexico to terms was that operating in the east. The campaign in this quarter began with an advance from Matamoras through Tamaulipas and Nuevo León into Coahuila. But as it progressed the plan was gradually assimilated, so far as these states were concerned, to that which had been followed in dealing with California and New Mexico, and became one of simple occupation; while the attack was shifted to the south, and the final advance was made from Vera Cruz direct on the city of Mexico.