 Complete the following phrase:  On 1 January 1956 Anglo-Egyptian Sudan declared independence as the Republic of Sudan; Chad followed later on 11 August 1960 and the border then became an international frontier between two independent states. Since then relations between the two states have often been tense, with each accusing the other of harbouring rebel groups which are allowed to cross the boundary. For instance, Sudan offered the FROLINAT rebel group sanctuary in western Sudan during the Chadian Civil War 19651979, and later the Patriotic Salvation Movement of current Chadian president Idriss Déby, who invaded Chad from his base in Darfur in 1990 and overthrew president Hissène Habré in the 1990 Chadian coup détat. Relations steadily deteriorated in later years as Chad became caught up in the War in Darfur, resulting in large numbers of refugees crossing the border. Following an attack on the Chadian border town of Adré in 2005 by Chadian rebel group the Rally for Democracy and Liberty, Chad publicly accused Sudan of backing the group. The subsequent 