Each country will be accountable for their terrorists. One terrorist attack here, will result in a equal random action there. If the war comes from a particular country, then that country must be responsible for the action of it's citizens upon other countries. After all, if an American citizen kills a Mexican, the rath of the Mexican government comes down on that citizen. The same goes for a group of people. Of course proof must be supplied to the United Nations before a counter attack is performed and the nation responsible for the original attack must be made aware and understand the result. Of course someone would need to keep score. After that point another Peace Treaty would be hmmered out.