Imagine, if you will, the Liberation of Paris in August 1944. Instead of General Charles de Gaul occupying the city on behalf of the Gouvernement Provisoire de la République Française, however, General Dwight D. Eisenhower rather congratulates the people of France on throwing off the yoke of German oppression to become the new 51st state of the Union. American forces passing through Canada and Australia on their way to other theatres slowly take defacto control over those dominions, while over the next few years, the contributions of British and Commonwealth forces to the war effort are quietly airbrushed out of the history books. Finally, as the nascent government of Clement Attlee emerges from the defeat of Churchill's coalition in 1945, US President Truman uses the ~2,000,000 US forces stationed in the UK to quietly depose him and establish the former United Kingdom as the new 52nd State of the Union.

This fantastical scenario may seem like a pure "what-if" writing prompt, but it's essentially what happens to the non-West Saxon parts of England in the 10th Century. I wrote an answer here about the concept of an Angelcynn and their Anglalond, from its conception in the 7th Century in the writings of Bede, to how the newly-unified country becomes England rather than Sæxland that is some interesting context. The salient point is that the West Saxon Cerdicing dynasty - also referred to as the Gewisse - have a frequently spectacular grasp of PR and the use of propaganda in expanding and consolidating political control.

Even before the large-scale arrival of the Danes in England, Wessex is an often aggressively expansionist kingdom. In 825, the Battle of Ellandun effectively transfers overlordship of the kingdoms of Kent and Sussex away from Mercia to Wessex; Ecgberht of Wessex wastes no time in installing his son Æthelwulf as King of Kent, a move repeated when Æthelwulf himself becomes king of Wessex and installs his son Æthelstan in Kent. In 830, Ecgberht even briefly exerts overlordship over Mercia before the two kingdoms settle into an uneasy peace that, surprisingly quickly, grows into an increasingly close alliance. The English political landscape of the 840s and 50s is marked by increasingly close cooperation and intermarriage between the two kingdoms, in which the Cerdicing biography The Anglo-Saxon Chronicle is keen to stress that Wessex was the dominant partner.

While they came close at times to disaster, the Anglo-Danish Wars of the 860s - 920s were, in many ways, a blessing in disguise for the Cerdicings. The defensive reforms carried out by Alfred in the 870s which see the establishment of the network of burh fortresses and fyrd garrisons sees a rapid expansion of royal bureaucracy and power, as well as providing an easily replicated means of rapidly and effectively expanding control over newly 'liberated' areas. Indeed, we see a hint of the new royal authority when Asser in his Vita Ælfredi suggests that a shire ealdorman could be replaced by Alfred not for any act of disloyalty or criminality, but for not taking steps to remedy a lack of literacy or numeracy.

The changing political landscape caused by the Danish conquests of Northumbria and East Anglia and the occupation of Eastern Mercia also opened a lot of doors for the West Saxons. In the late 870s and early 880s, Alfred of Wessex is able to establish what appears to have been a fairly extensive de facto control over the major Mercian port site at Lundenwic, even while pursuing a close alliance with the Mercian king Ceolwulf II (who would then be demonised in West Saxon propaganda). When Ceolwulf is killed while campaigning in Wales in 881, Alfred is able to exploit the Mercian succession to extend political control over Mercia. The new Mercian leader, Æthelred, appears to have accepted West Saxon overlordship over Mercia in return for Alfred's support, and marriage to Alfred's daughter Æthelflæd, ruling not as Rex Merciorum but as subregulus,, princeps or procurator. It was likely intended that Æthelflæd, as a dutiful Cerdicing daughter, would ensure that Mercia passed under the overlordship of her father, or more likely her brother or nephew, when her husband's rule came to an end. This was a rare misstep on the part of Alfred: Æthelflæd appears to have embraced her new country fervently, turning her considerable political talents to bolstering Mercia's armies, constructing a network of burhs to mirror that of Wessex, establishing a hugely important economic centre at Chester, rebuilding a new capital at Gloucester, enthusiastically encouraging the patronising of local Mercian saints' cults, establishing an independent Mercian historical chronicle, and leading expansionist Mercian campaigns into both Wales and the Danelaw. Ultimately, she left the rule of Mercia to her own teenage daughter rather than to her West Saxon brother, Edward the Elder, although this was ultimately not to last, with Ælfwynn being deposed by her uncle only a few months into her rule. Nonetheless, her fostering of her nephew, Æthelstan (at least according to William of Malmesbury), meant that Mercia went into his eventual unification of England on a much more even footing, even if it was one almost entirely overlooked by our surviving West Saxon historical sources.

We get a good glimpse of Wessex's expansionist ambitions in the Anglo-Saxon Chronicle for 886 when, after reestablishing the Roman defences at London and relocating Lundenwic within them, Alfred receives the political submission of:

all of the English not under the yoke of the Danes.

This is a carefully chosen use of language, with Bede's concept of Angelcynn strictly to the fore. While Asser might continue to refer to The Christians versus The Heathens, the Anglo-Saxon Chronicle increasingly frames Alfred's wars as "the English" versus the Danes. The phrasing of Alfred's overlordship is notable for its complete lack of reference to the East Angles or the Northumbrians; these are no longer independent kingdoms with their own long histories and periods of political dominance. Now, under Danish control, they are rhetorically reduced to 'English' in need of liberation. That that 'liberation' throughout the 910s and 920s was most often the exchange of Danish overlordship for West Saxon overlordship can be conveniently overlooked by West Saxon sources. From the early 910s onwards, Wessex and Mercia slowly but surely expand their frontiers into the Danelaw, expanding their networks of burh fortresses to rapidly consolidate control over newly 'liberated' areas, until Æthelflæd's Mercian army receives the submission of York in 918.

Turning the loose coalition of alliances and overlordship inherited by Æthelstan in 924 into a coherent unified kingdom was a long and frequently bloody process that would last until the 940s and arguably wouldn't be complete until the reign of Edgar in the 970s (who still recognised Northumbrian pseudo-autonomy), but this nonetheless established the foundations of a single unified country.