This is a huge question, so I might not be able to cover every facet of it, but I will do my best to summarize the contributions of Nikephoros II Phokas to the betterment of the Byzantine Empire in the 10th Century.

Let us start with his origin. Nikephoros came from a well-known family of Cappadocian landowners and generals who had a long history of proud service to the Empire. Nikephoros' grandfather, Nikephoros Phokas the Elder, was one of the most successful generals of the early Macedonian period, leading several important expeditions against the Arabs and reasserting Byzantine control over Southern Italy. Nikephoros II's father, Bardas Phokas, the Domestikos ton Scholon (Domestic of the Schools - in other words, Supreme Commander), was also a highly-regarded general serving under Constantine VII Porphyrogennetos.

Following the proud military tradition of his family, Nikephoros and his younger brother Leo enlisted at an early age in the Byzantine army and gradually rose through the ranks to become Strategoi of the Cappadocian and Anatolikon themes by their 30s. Nikephoros, as the older and more experienced brother, was promoted to the high-ranking position of Domestic of the Schools after his father was wounded and defeated whilst fighting the Hamdanids in AD 953 at the Battle of Marash. Leo was also eventually raised to this position.

Throughout the 950s and early 960s, Nikephoros gained a reputation as a military genius. The retaking of Crete from the Muslims, a plan envisioned and unsuccessfully carried out in AD 911 and again in AD 949, first by Leo VI 'the Wise' and his son Constantine VII, was renewed by Constantine's son, Romanos II, in AD 959. Leading a fleet of some 300 ships armed with the greatest weapons the Empire had to offer, Nikephoros personally led his troops ashore and won a brilliant victory by storm during the Siege of Chandax, a feat which he was well known for within his lifetime. Within 2 short years, much to the benefit of the Empire, the conquest of Crete was complete, finally securing the Aegean Sea as "a Byzantine lake" after several centuries of Arab dominance.

Following his return from the Cretan Expedition, Nikephoros marched to Syria to strike a further blow against the Arabs, who had been launching annoying incursions into Anatolia (as they had done for centuries). He aimed to take the plain of Cilicia (the crossroads of Southern Asia Minor, and one of the primary routes into Byzantine lands from the East), and to eliminate the Islamic stronghold of Aleppo. Using a new set of deceptive tactics, Nikephoros and John Tzimiskes (his equally brilliant nephew) were able to inflict severe defeats on the Arab armies. Drawing upon experience gained during these campaigns, Nikephoros started writing his famous military treatises, the most famous of which is the Praecepta Militaria - a treatise focusing on the use of kataphraktoi.

One tactic developed during this time that was especially noteworthy was Nikephoros' use of a feigned retreat to prevent raiders from gaining access to the wide open land of the plateau of Anatolia. The open land of the plateau allowed Arab raiders to strike any number of Byzantine cities once they had gotten through the mountain passes. Nikephoros, recognizing an inherent weakness in these tactics, devised a plan to lure raiders into the plain, but to trap them with three or more sets of flanking formations that would close in and surround them. When the raiders emerged from the passes, the flanking battlegroups would emerge from their hiding spots and begin to converge from the front and sides, while another group blocked the retreat back through mountain pass, providing no means of escape and also preventing the raiders from dispersing. In this way, the various Byzantine battlegroups would close in and either capture or massacre the raiders. It was for these devious tactics, as well as his incredible track record on the battlefield, that Nikephoros earned the nickname "The Pale Death of the Saracens". On the battlefield, Nikephoros only had but one equal, his nephew and later rival, John Tzimiskes.

The details of Nikephoros' accession are interesting. Emperor Romanos II died suddenly at the age of 26 in AD 963. Romanos, like his father, was probably a sickly man, and this was made worse by the fact that he was rather hedonistic. It has been posed that it was the Empress Theophano that killed him via poison in a bid for power. Whatever the case, Romanos left two sons, Basil and Constantine (both future emperors), but they were not of age, and could not yet assume the throne. Siezing the opportunity, Nikephoros, being the foremost general in the Empire, marched on Constantinople with the support of the Empress and the Patriarch and assumed the Emperorship by the middle of the year in AD 963. Although Nikephoros had taken an oath of chastity after the death of his first wife, his desire for power drove him to break the oath and marry the widowed Empress Theophano (a great beauty), even though it seems he wasn't that interested in her personally. Due to the popularity of the Macedonians, his wife, and perhaps his personal morality, Nikephoros did not depose the young Emperors and seems to have cared for them.

As Emperor, Nikephoros continued to be seen as a great military leader. He smashed the Islamic states in Mesopotamia and Syria, effectively eliminating them as a threat to Byzantine sovereignty for the rest of the 10th Century. Due to the capture of Crete and the great successes on the mainland, Cyprus, an ancient Byzantine holding taken by the Arabs, was once again brought under the Imperial banner. The ravaging of Bulgarian lands by the Kievan Rus (incited by Byzantine diplomacy) allowed Nikephoros to continue to focus on his Eastern campaigns. To allow further focus on the East, the Balkan frontier continued to be secured by Nikephoros' brother Leo. It had been many centuries since the Empire had seen the borders so secure and been poised to make so many gains.

As an administrator, Nikephoros seemed capable enough, but due to his pious nature and military mindset, he seems to have made quite a few enemies. Nikephoros was austere, not willing to spend on things that he deemed frivolous and his economic plans diverted funds primarily to his military campaigns, while neglecting many other realms of Byzantine society which furthered his unpopularity. Liutprand of Cremona, a Western visitor to Constantinople during this time had very little good to say of Nikephoros, seemingly because of his distant and unlavish nature. Liutprand also remarks that Nikephoros was rather unsightly, although, it is hard to say whether this is just a further jab at the Emperor, but if true, it might've also contributed to his unpopularity. Nikephoros' extreme piety also made him seem distant and severe to his contemporaries (especially in comparison to his nephew John Tzimiskes). Nikephoros had few real friends, a fact which most certainly played a role in his assassination.

Ultimately, Nikephoros II Phokas is a leader who did a great deal for Byzantium. Coming from a martial background, Nikephoros greatly furthered the military legacy of the Macedonian dynasty. While previous Emperors had done a great deal to develop the intellectual side of the economic, governmental, and military aspects of the Empire, not since the time of Basil I had the Macedonians had a true soldier-emperor willing to lead from the front. Nikephoros consistently led the Empire to victory during his military career, inflicting severe defeats upon the Saracens and allowing the Empire to flourish. His steady leadership, while primarily militarily focused, set the Empire up for additional gains under John I and Basil II, removed significant threats from the borders, and once again secured the Empire's position as the superpower of the Mediterranean after centuries of shameful defeats and muted glory. For these reasons, Nikephoros II is certainly to be considered one of the greatest leaders in Byzantine history.