Undeclared war is also a war. War starts by aggression. Thats what Syria did in 1948.
Article 3
Any of the following acts,
regardless of a declaration of war, shall, subject to and in accordance with the provisions of article 2, qualify as an act of aggression:
(a) The invasion or attack by the armed forces of a State of the territory of another State, or any military occupation, however temporary, resulting from such invasion or attack, or any annexation by the use of force of the territory of another State or part thereof,
(b) Bombardment by the armed forces of a State against the territory of another State or the use of any weapons by a State against the territory of another State;
(c) The blockade of the ports or coasts of a State by the armed forces of another State;
(d) An attack by the armed forces of a State on the land, sea or air forces, or marine and air fleets of another State;
(e) The use of armed forces of one State which are within the territory of another State with the agreement of the receiving State, in contravention of the conditions provided for in the agreement or any extension of their presence in such territory beyond the termination of the agreement;
(f) The action of a State in allowing its temtory, which it has placed at the disposal of another State, to be used by that other State for perpetrating an act of aggression against a third State;
(g) The sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State of such gravity as to amount to the acts listed above, or its substantial involvement therein.
http://www.un-documents.net/a29r3314.htm