Another aspect of the situation concerned it more seriously. On the day Congress passed the war bill (May 13, 1846) orders were issued to blockade the ports of Mexico. Several definite aims prompted this action. Primarily, of course, it was desired to prevent supplies of all kinds from reaching the enemy, and to deprive them of the almost indispensable revenues obtained in peace by taxing imports; but there were also hopes that loss of business would induce Great Britain and France, which had a profitable trade in that quarter, to urge upon Mexico the acceptance of our terms. The blockade was therefore to be enforced vigorously. At the same time neutrals were to be treated with all reasonable indulgence. Theoretically only their war vessels had the right of entering closed ports, but practically the intention was to broaden that narrow door considerably. Toward itself, however, the United States determined to be strictly faithful in observing its declared principles. Merely those ports where the order could become effective were in view. The announcement of blockade was to be made as public as it could be; and in particular the government required that a full warning should be given to neutral ships.[30.8] For the work thus imposed upon him Conner had ample time to prepare. As early as August, 1845, he was directed to blockade the Gulf ports in case of war; early in 1846 he knew of Mexico's attitude regarding Slidell; before the end of March his vessels occupied convenient positions; and promptly on the outbreak of hostilities a blockade was announced at the chief harbors. By July, with some assistance from the revenue service, it extended from the Rio Grande to the Goatzacoalcos.[30.9] But the difficulty of making it continuously effective proved to be extraordinary. There were not vessels enough of the proper kind; occasionally a more or less complete concentration became necessary; and accidents of many sorts occurred. Uncharted shoals and rocks, currents of unknown direction and force, the frequent haze, and the darkening of the lighthouses made extreme caution imperative. The suddenness and violence of the storms almost surpassed belief. At Vera Cruz the _Somers_ was blown over and sunk before Semmes, her able commander, could take steps to avert the disaster. Even at the anchorages one would suddenly hear on a calm afternoon the clarion orders of the speaking trumpet; the ship would quiver and reverberate as the cable of the heaviest anchor ran swiftly out; in a moment the storm would burst; and for days it might be a question almost hourly of going ashore. At such times all sailing vessels on patrol duty had to make instantly for the open sea, and before they could return to their stations a lurking blockade runner could perhaps enter the port. Owing to such difficulties Alvarado and Frontera, for instance, could not be watched continuously.[30.9]