 Complete the following phrase:  During the entire course of his campaign in Dalmatia and Croatia, Tvrtko was also engaged in skirmishes in the east of his realm, which prevented him from focusing all of his manpower on expansion westwards. The Kingdom of Bosnia was believed to be far from the reach of the Ottomans during Tvrtko Is reign, shielded by a belt of independent Serbian statelets. George II of Zeta, however, purposely enabled the Turks to launch raids against Bosnia, first in 1386 of which little is known and again in 1388. In the second instance the Ottoman and Zetan invaders, led by Lala Şahin Pasha, penetrated as far as Bileća. The 