The Law of Nations or the Principles of Natural Law (1758)Emmerich de Vattel Of the Enemy's Allies — of Warlike Associations — of Auxiliaries and Subsidies§ 78. Treaties relative to war.§ 79. Defensive and offensive alliances.§ 80. Difference between warlike associations and auxiliary treaties. § 81. Auxiliary troops.§ 82. Subsidies.§ 83. When a nation is allowed to assist another.§ 84. and to make alliances for war.§ 85. Alliances made with a nation actually engaged in war.§ 86. Tacit clause in every warlike alliance.§ 87. To refuse succors for an unjust war is no breach of alliance. § 88. What the casus fœderis is.§ 89. It never takes place in an unjust war.§ 90. How it exists in a defensive war.§ 91. and in a treaty of guarantee.§ 92. The succor is not due under an inability to§ 93. Other cases.§ 94. Refusal of the succors due in vir-§ 95. The enemy's associates.§ 96. Those who make a common cause with the enemy are his associates § 97. And those who§ 98. Or who are in an offensive alliance with him.§ 99. How a defensive alliance as-§ 100. Another case.§ 101. In what case it does not produce the same effect.§ 102. Whether it be necessary to declare war against the enemy's associates.
     1.    Book II. chap. xvii.
     2.    See Wolf, Jus Gentium. §§ 730 and 737.
     3.    De Jure Belli et Pacis, lib. iii. cap. iii. § 10.
     4.    Livy, lib. xxxviii.
     5.    Grotius, ubi supra, not. 3.