The latest updates from the crisis in Ukraine have unfolded new dimensions in international law. There is no dispute over the fact that the autonomous peninsula of Crimea has strong ties to Russia, not only because of its geographical proximity but even because of the predominant Russian-speaking population. Moreover Russia controls a military base in Sevastopol which for the time being is considered to destabilize the region.
In addition to that, former President Yanukovych approved in year 2010 the extension of the leasing contract for the Russian Black Sea Fleet over the facilities of the peninsula for another 25 years starting in 2017 against a reduction of the Russian gas prices. Thus Russia has been carefully moving towards the enforcement of its geostrategic interests in Ukraine.

In the last days Russia has shown its leverage by undertaking a military intervention in Crimea without any prior authorization from either the interim Ukrainian government or the Security Council. In this respect Article 2 (4) of the UN Charter states that:

"All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations."

Obviously Russia is breaching international law rules by making a unilateral intervention in Crimea without any prior authorization from Ukraine, thus interfering with its territorial and political integrity. The International Court of Justice has made clear the principle of non-intervention in the case of Nicaragua v. United States of America by stating that: "The principle of non-intervention involves the right of every sovereign State to conduct its affairs without outside interference".

The arguments of Russia in defending its actions are the protection of Russian citizens in Crimea (protection of nationals abroad is a right of states under international law). Up to now there wasn't any evidence that Russian nationals in Crimea have been subjected to any violence through Ukrainian authorities nor that their lives were put in danger. Russia has not only sent its troops and armoured vehicles but also Russian ships have been seen to move in Crimea, so basically there seem to exist some significant evidence of thinking of a deliberate takeover of the peninsula from Russia.

Art 51. of the UN Charter foresees the right of self-defence for a member country if it is under an armed attack from another country. Ukraine might rely on Art. 51 and call for self-defence, but in doing that, it needs to prove that an armed attack against it has taken place. Is it the case here? Is Ukraine under an armed attack from Russia?  The qualification of an armed attack has been delivered by the ICJ in the Nicaragua Case: "...this is to be understood as meaning not merely action by regular armed forces across an international border, but also the sending by a State of armed bands on to the territory of another State..."

On the other hand under the light of customary international law Ukraine might have resorted to the right of "preemptive self-defence" which could be invoked in case of an existent imminent threat as opposed to the right of self-defence contemplated in Art.51 of the UN Charter, which appears to be more rigid in terms of its use. Nevertheless when employing preemptive self-defence the following elements need to be fulfilled: "necessity of self-defence, instant, overwhelming, leaving no choice of means and no moment of deliberation."  The United Nations have put limits to this right through Art 51. by requiring that self-defence should only be employed when there is an existent armed attack.

But once again the question is whether there is an armed attack in Ukraine and in case Ukraine reacts to the Russian intervention, what should it bear in mind before employing any use of force against Russian troops?