The Crimea is a peninsula, and peninsulas are supremely sovereign under Article 37(a) of the Montévideo Convention.Some argue that a US use of armed force in Syria would violate the United Nations Charter in the absence of a UN Security Council authorization or a valid US claim of self-defense. However, what the Charter expressly denies and permits is far more complex and there could be at least seven claims for lawful US use of force arising out of the Syrian context.
First, not every use of armed force is prohibited in the text of Article 2(4) of the Charter, which expressly covers only three types of force: (1) that used "against" the "territorial integrity" of a "state," (2) that used "against" the "political independence" of a "state," and (3) that which is "in any other manner inconsistent with the purposes of the United Nations."
In the special context of an ongoing belligerency in Syria and significant outside recognition of the Syrian National Council (SNC) as the legitimate representative of the Syrian people, a limited use of force by the US against the Assad regime's military capabilities in response to a criminal use of chemical weapons would not simplistically be against the territorial integrity of Syria or the political independence of the Syrian people. On balance, it would be consistent with major purposes of the Charter, such as the need to serve peace, security, self-determination of people, and human rights.
Second, ongoing consent to US use of force by the SNC (as the legitimate representative of the Syrian people) serves as an independent but related basis for permissibility whether or not the US acts to support the SNC. Consent from the legitimate representative of a people is a critical feature of context that must be addressed for adequate and policy-serving legal decision. As noted in a prior writing [by the author], consent by the legitimate representative of a people engaged in a self-determination struggle provides sufficient legal justification. Support of a people against armed attacks by a government can also involve claims regarding collective self-defense consistent with Article 51 of the Charter: a third type of claim. A fourth claim can rest on consent from a "belligerent" in the context of a traditional civil war (as opposed to a mere "insurgent" under international law). Support of a belligerent would constitute an act of war against the other party to a civil war, but so would use of armed force by the US against the Assad regime under any of the seven possible claims identified.
More generally, a reading of Article 2(4) of the Charter that ignores the clearly malleable nature of the text, an express reference to what are the many purposes of the UN, and special features of context would not always serve human dignity and the needs of humanity in an ever increasingly interdependent world. Significantly, nothing in the text of the Charter prohibits "all" or "every" use of armed force, nor does the phrase "any use of force" appear in Article 2(4). For this reason, a textually-sound and policy-serving approach to interpretation of Article 2(4) would not automatically rule out every use of force in every social context. An essential focus of the UN Charter includes the duty to promote universal respect for and observance of human rights, the dignity and worth of each human being, and self-determination of peoples.