Saul's death left Israel at the mercy of the Philistines and internally divided. David, who had been in the somewhat questionable position of a vassel of Achish, king of Gath, now moved to Hebron, where he was anointed king of Judah (2 Sam.2:1-4). But at Mahanaim east of Jordan, Saul's son Eshbaal (ishbosheth) was established as king. The real power in his attenuated kingdom seems to have been Abner, Saul's commander-in-chief (2 Sam.2:8ff).
David's advance to authority over all Israel was facilitated by a grim sequence of personal and family feuds. After an encounter between a force of David's men under Joab and a similar party led by Abner, Asahel, Joab's brother, pursued Abner but was killed by him. This created a blood feud between Abner and Asahel's brothers, Joab and Abishai (2 Sam.2). Later, Abner quarrelled with Ishbosheth and offered David his support, promising to secure for him dominion over all Israel. On hearing of the agreement, Joab hurried after Abner and treacherously murdered him, thus removing the one really strong leader of the central tribes (2 Sam.3). Some time afterwards Ishbosheth was assassinated (2 Sam.4) and the tribes which until then had retained their allegiance to the house of Saul now offered the kingship to David. In a remarkable way the field had been cleared for him by the violence comitted by others, from which he had publically disssociated himself.
The extension of David's royal authority over Israel was established by a covenant (2 Sam.5:3). It has been claimed that no real unity was established between Judah and the central and northern tribes other than that which was maintained by David's perssonal authority. Later events such as Absalom's revolt, and, even more decisively, the disruption of the kingdom, show that serious tensions remained. But at all events the unity seems to have been sufficiently effective to rouse the Philistines to action. David hasd been a vassal of the Philistines. As king of Judah he might eprhaps be disregarded. As king of Judah and Israel he was a serious threat to Philistine power. They moved against him, but were twice defeated (2 Sam.5:17-25). Such unity as had been achieved was reinforced by the winning of independence from foreign domination.
It seems likely that these victories preceeded the capture of Jerusalem (2 Sam.5:4-10) though they follow it in the narrative. Not only was it an outstanding military feat, but it marked a further step in the consolidations of David's royal authority and the unification of his kingdom. It provided him with a centre from which he could exercise authority over the country far more effectively than from Hebron in the remote south. Jerusalem lay between the territory of Judah and that of the central and northern tribes, and it had belonged to neither. The choice of this city as the new capital was a shrewd move. David further enhanced its prestige by bringing the ark from Kiriath-jearim and installing it in a sacred tent at Jerusalem (2 Sam.6). in this way he established a powerful link between the hitherto non-Israelite city and Israel's religious traditions and so laid the foundation of the later religious dominance of Jerusalem.
By a succession of victorious campaigns (2 Sam.8:10-11) David further strengthened the position of his kingdom, providing it with a semicircle of buffer territories and tributary states and gaining control over several lucrative trade routes. The Philistines had already been quelled. East of the Dead Sea, the Moabites were subdued. A campaign against Ammon proved difficult, partly because of the help given to the Ammonites by the Aramaeans in the north. Only when the Aramaeans had been defeated, and after a costly siege, was the Ammonite capital taken. Victory over the Edomites was follwoed by ruthless measures against the royal house and the population, and by the establishment of a provincial administration. Control of this territory gave access to the Red Sea, and also to improtant mineral resources. Further operations were undertaken against the Aramaeans and as a result of David's sway, and the area from which he received tribute, the borders were extended to Damascus and beyond. Also in the north, but further west, amicable relations were established with Phoenicia (2 Sam.5:11), a link which was to prove important to Solomon's reign and the later history of Israel.
Thus, at a time when there was no great power to intervene (Egypt and the Mesopotamian kingdoms were weak), David established Israel as an independent state with extensive dependencies, with access to important natural resources (e.g. copper and iron), and with influential commercial connections. Within a single generation there was a striking advance in the general standard of living, a development which was impressively continued in the next reign. A royal administrative system began to be organised on Egyptian lines; and in the military field a standing force of foreign mercenaries (Cherethites and Pelethites - 2 Sam.8:18, 20:23) was maintained, in addition to the Israelite militia (the army - 2 Sam.8:16; 20:23). The king also had a personal body guard. David's toyal household was considerably more pretentious than that of Saul, though modest as compared with that of his successor.
Thus David's success in building an empire can be attributed to a complex of events, some out of his control, and others directly as a result of his initiative. In summary, the ajor reasons for David's success were the fact that he was a good military tactician, he was politically shrewd, he obtained economic power through his conquests, he was courageous, and the great nations of the time (Egypt, Babylon and Assyria) were weak. Last but not least, he had Yahweh on his side, for without Him, David would probably have been swept away by forces completely out of his control. Only an Omnioptent Elohim (God) could have organised so many different conditions in such a way to enable a tiny and unimportant country to suddenly reach such imperial heights through a devoted servant, David.