http://www.usslibertyinquiry.com/law/jag7.html
"7.1 INTRODUCTION

The law of neutrality defines the legal relationship between nations engaged in an armed conflict (belligerents) and nations not taking part in such hostilities (neutrals). The law of neutrality serves to localize war, to limit the conduct of war on both land and sea, and to lessen the impact of war on international commerce.

Developed at a time when nations customarily issued declarations of war before engaging in hostilities, the law of neutrality contemplated that the transition between war and peace would be clear and unambiguous. With the advent of international efforts to abolish "war," coupled with the proliferation of collective security arrangements and the extension of the spectrum of warfare to include insurgencies and counterinsurgencies, armed conflict is now seldom accompanied by formal declarations of war. Consequently, it has become increasingly difficult to determine with precision the point in time when hostilities have become a "war" and to distinguish belligerent nations from neutrals. Notwithstanding these uncertainties, the law of neutrality continues to serve an important role in containing the spread of hostilities, in regulating the conduct of belligerents with respect to nations not participating in the conflict, in regulating the conduct of neutrals with respect to belligerents, and in reducing the harmful effects of such hostilities on international commerce.

For purposes of this publication, a belligerent nation is defined as a nation engaged in an international armed conflict, whether or not a formal declaration of war has been issued. Conversely, a neutral nation is defined as a nation that has proclaimed its neutrality or has otherwise assumed neutral status with respect to an ongoing conflict.
7.2 NEUTRAL STATUS

Customary international law contemplates that all nations have the option to refrain from participation in an armed conflict by declaring or otherwise assuming neutral status. The law of armed conflict reciprocally imposes duties and confers rights upon neutral nations and upon belligerents. The principal right of the neutral nation is that of inviolability; its principal duties are those of abstention and impartiality. Conversely, it is the duty of a belligerent to respect the former and its right to insist upon the latter. This customary law has, to some extent, been modified by the United Nations Charter (see paragraph 7.2.1).

Neutral status, once established, remains in effect unless and until the neutral nation abandons its neutral stance and enters into the conflict."

II. Las fuentes del Derecho de la neutralidad.


http://www.enciclopedia-juridica.biz14.com/d/neutralidad/neu...
"El Derecho de la neutralidad fue surgiendo como derecho consuetudinario. La neutralidad marítima se desarrolló ya en la Edad Media. En cambio, el Derecho positivo de la neutralidad en la guerra terrestre se ha elaborado mucho más tarde."

http://www.lawofwar.org/Neutrality.htm
"...the collective systems of the League of Nations and the United Nations allow for contingencies in which member States--let alone non-member States--remain free to decide for themselves on their attitude to armed conflict. Moreover, in armed conflicts inside a country, third States, by their recognition of revolutionaries as belligerents, may still become obligated to apply the law of neutrality. Finally, there are States, such as Switzerland, which even now are under an international obligation to remain permanently neutral and, more recently, Austria has chosen to base her own foreign policy on the principle of neutrality. Thus, the law of neutrality appears to be far from dated."

Según entiendo, entonces, el derecho de la neutralidad tiene fuente consuetudinaria o legislativa.