The 1919 Versailles Peace Treaty between Germany and the Allied Powers that concluded World War I precipitated the next phase in the development of modern terrorism. The Covenant of the League of Nations (the League Covenant) redistributed former German and Turkish colonies and other dependencies through the League mandate system, which was designed to ensure a "mild form of international accountability for [their] administration" (Thullen, 1964, p. 9). Otherwise, standards to help integrate minority peoples in the new States created after 1919, such as Yugoslavia, were also provided for, and were intended to serve a peace-making function, while deterring transboundary alliances (Veatch, 1983/2010, p. 369). Nonetheless, protected common rights, such as the right to a nationality, the free exercise of belief, employment and identity, and rights enshrined in the ideals of the mandate system and policies on minorities did not apply to the peoples and minorities in the victorious States, which later were instead the guarantors of peace and security as a whole, while the victors also presumably rejected the notion of new colonial acquisitions when declining to annex former colonies and non-governing territories. Overall, the Covenant system of mutual defence appeared to be designed to favour international security concerns rather than those of the rule of law and international law, in the event of a conflict.
The twentieth century link between modern terrorism and the ideal of self-determination arose within the competing ideologies of communist/socialist theory (Lenin, 1914/1972), and those reflected in a League of Nations Covenant in which there is no express reference to the principle of self-determination. In contrast to the rejection of all prior Czarist debts and obligations by the post-war revolutionary Government of the Soviet Union, the United States, under the presidency of Woodrow Wilson, was a strong proponent of the "peoplehood" principle of self-determination (Morgan, 1980, pp. 355-359). However, Wilson did not hold sway on this concept of self-determination at the Versailles conference at which the final version of the Covenant was agreed. In addressing the issue of self-determination, article VI of the Wilson-Miller draft of the proposed League Covenant provided that:
The League of Nations shall require all new states to bind themselves, as a condition precedent to their recognition as independent or autonomous states, to accord to all racial or national minorities within their jurisdiction exactly the same treatment and security, both in law and in fact, that is accorded to the racial or national majority of their people (Fawcett, 1979, p. 7).
Even so, such issues in the era of the League of Nations represented only a number of the factors to be considered during the formation of new States, whether within the operational context of mechanisms to attain statehood, or as a matter of self-help. When issues relating to self-determination arose early on in the League's existence during the Aaland Islands dispute in 1920 between Sweden and Finland, the League Council appointed the International Commission of Jurists to determine the matter. The Commission concluded that the mere recognition of the principle of self-determination, as made out in a number of treaties, did not create a positive rule of the law of nations (Wilson, 1988, p. 57). In part, this was due to the Commission of Jurists’s apprehension about creating a precedent for secession, thereby encouraging anarchy. However, a subsequent Committee of Inquiry refined this result by concluding that if Finland failed to provide the islanders with certain specified guarantees, they would indeed have a right under international law to a plebiscite, which could have resulted in separation from Finland. Nowadays, the Aaland Islands solution is regarded as a precedent for successful international dispute settlement (O'Brien, 2012).
In the meantime, the spate of terrorist assassinations continued. By the 1930s, several bilateral agreements referred to the suppression of terrorism, and many extradition treaties contained clauses excluding assassination attempts against Heads of State from the exempted list of political offences (e.g., Convention on Extradition 1933, article 3 (e)). The assassinations of King Alexander I of Yugoslavia and the French Minister for Foreign Affairs together in Marseilles on 9 October 1934 brought matters to a head, when the requested extradition of the persons accused was refused by Italy on the grounds that the offences were political (Chadwick, 1996). In response, a Committee of Experts was established by the League Council to draft a Convention on Terrorism for the establishment of an International Criminal Court, which would have jurisdiction over certain acts specified as acts of terrorism in the Convention, and which States Parties were obliged to criminalize within their national laws. Article 1(2) of the Terrorism Convention defines "acts of terrorism" as "criminal acts directed against a state" (1937). Such acts must be "intended or calculated to create a state of terror in the minds of particular persons, or a group of persons or the general public". The Convention is silent on the purpose of the fear generated (Chadwick, 1996).
Nonetheless, State and regional traditions of asylum, coupled with strong national sympathies, made any differentiation between "terrorist" and "political" offences problematic, and the granting of asylum remained possible then, as now. Moreover, the 1937 Terrorism Convention, as a creature of its time, had no "international" criminal law to be grounded in, as reflected in article 19, which provides that:
The present Convention does not affect the principle that, provided the offender is not allowed to escape punishment owing to an omission in the criminal law, the characterisation of the various offences dealt with in the present Convention, the imposition of sentences, the methods of prosecution and trial, and the rules as to mitigating circumstances, pardon and amnesty are determined in each country by the provisions of domestic law.
Therefore, the law applicable to any criminal prosecution for acts established as offences under the Convention was to be that of the referring, and thus, prosecuting, State. Unfortunately, World War II erupted soon after, and neither convention entered into force.