 Complete the following phrase:  Due to the ongoing conflicts with the Arabs during the 670s, Constantine had been forced to conclude treaties in the west with the Lombards, who had captured Brindisi and Taranto. Also in 680, the Bulgars under Khan Asparukh crossed the Danube into nominally Imperial territory and began to subjugate the local communities and Slavic tribes. In 680, Constantine IV led a combined land and sea operation against the invaders and besieged their fortified camp in Dobruja. Suffering from bad health, the Emperor had to leave the army, which panicked and 