Russia's invasion? The First Chechen War was a domestic conflict as far as the Russians were concerned. The Chechens were separatists. Chechnya was not a sovereign nation-state and enjoyed no international recognition. If anything, the war was a Russian law enforcement operation (again, legally). It's worth keeping in mind that all the Chechen fighters were either ex-Soviet Army, ex-Russian Army or ex-Russian law enforcement agents (this last point actually still holds true to this day; just look up the backgrounds of any high-profile Chechen military leader). Then as now, it is probably most accurate to view the Chechen conflicts as a civil dispute (at least from a legal standpoint). You would have to go back to the mid-19th century for the last time that Chechnya could be viably considered a non-Russian territory.

Thus the entire concept of casus belli simply doesn't apply here. On the other hand, if you mean "invasion" strictly in the operational sense, then I would simply point out that, as a law enforcement operation, the First Chechen War was a perfect example for why the army should not be used in such a capacity. Law enforcement in Chechnya is actually a very interesting topic, but the Russians understood it poorly in '94, and I would have to violate the 20-year rule to discuss it in any depth or detail. Nevertheless, I urge you to think of Chechnya, as well as Russia's affairs in the North Caucasus, in terms of politics and security rather than military action--if anything because these conflicts have taught us that applying military thought to such situations (as the Russian leadership did repeatedly) leads to quite horrible outcomes.

EDIT: To be a little more specific: The proper conceptualization of the First Chechen War lies more in the struggle between a federal center and a region that wishes to secede (and is trying to do so unlawfully under the domestic law in power), than as a conflict between two sovereign territories (in which one or the other is an unlawful aggressor under international law).