The external relations of a people may be compared to those of private individuals, and they cannot be advantageously maintained without the agency of the single head of a government. The exclusive right of making peace and war, of concluding treaties of commerce, of raising armies, and equipping fleets, was therefore granted to the Union.[127] The necessity of a national government was less imperiously felt in the conduct of the internal affairs of society; but there are certain general interests which can only be attended to with advantage by a general authority. The Union was invested with the power of controlling the monetary system, of directing the post-office, and of opening the great roads which were to establish communication between the different parts of the country.[128] The independence of the government of each state was formally recognized in its sphere; nevertheless the federal government was authorized to interfere in the internal affairs of the states[129] in a few predetermined cases, in which an indiscreet abuse of their independence might compromise the security of the Union at large. Thus, while the power of modifying and changing their legislation at pleasure was preserved in all the republics, they were forbidden to enact _ex post facto_ laws, or to create a class of nobles in their community.[130] Lastly, as it was necessary that the federal government should be able to fulfil its engagements, it was endowed with an unlimited power of levying taxes.[131] In examining the balance of power as established by the federal constitution; in remarking on the one hand the portion of sovereignty which has been reserved to the several states, and on the other the share of power which the Union has assumed, it is evident that the federal legislators entertained the clearest and most accurate notions on the nature of the centralisation of government. The United States form not only a republic, but a confederation; nevertheless the authority of the nation is more central than it was in several of the monarchies of Europe when the American constitution was formed. Take, for instance, the two following examples:-- Thirteen supreme courts of justice existed in France, which, generally speaking, had the right of interpreting the law without appeal; and those provinces, styled _pays d'etats_, were authorized to refuse their assent to an impost which had been levied by the sovereign who represented the nation. In the Union there is but one tribunal to interpret, as there is one legislature to make the laws; and an impost voted by the representatives of the nation is binding upon all the citizens. In these two essential points, therefore, the Union exercises more central authority than the French monarchy possessed, although the Union is only an assemblage of confederate republics.