Allegiance is the tie which binds the subject to the Sovereign in return for that protection which the Sovereign affords the subject. It was the mutual bond and obligation between monarch and subjects, whereby subjects are called his liege subjects, because they are bound to obey and serve him; and he is called their liege lord, because he should maintain and defend them (Ex parte Anderson (1861) 3 El & El 487; 121 ER 525; China Navigation Co v Attorney-General (1932) 48 TLR 375; Attorney-General v Nissan  1 All ER 629; Oppenheimer v Cattermole  3 All ER 1106). The duty of the Crown towards its subjects is to govern and protect. The reciprocal duty of the subject towards the Crown is that of allegiance.
At common law allegiance is a true and faithful obedience of the subject due to his Sovereign. As the subject owes to his king his true and faithful allegiance and obedience, so the Sovereign is to govern and protect his subjects, regere et protegere subdititos suos, so as between the Sovereign and subject there is:
Natural allegiance and obedience is an incident inseparable to every subject, for as soon as the Sovereign is born, they owe allegiance and obedience (Ex parte Anderson (1861) 3 El & El 487; 121 ER 525). Natural-born subjects owe allegiance wherever they may be. Where territory is occupied in the course of hostilities by an enemy's force, even if the annexation of the occupied country is proclaimed by the enemy, there can be no change of allegiance during the progress of hostilities on the part of a citizen of the occupied country (R v Vermaak (1900) 21 NLR 204 (South Africa)).
Allegiance is owed both to the Sovereign as a natural person and to the Sovereign in the political capacity (Re Stepney Election Petition, Isaacson v Durant (1886) 17 QBD 54 (per Lord Coleridge CJ)). Attachment to the person of the reigning Sovereign is not sufficient. Loyalty requires affection also to the office of the Sovereign, attachment to royalty, attachment to the law and to the constitution of the realm, and he who would, by force or by fraud, endeavour to prostrate that law and constitution, though he may retain his affection for its head, can boast but an imperfect and spurious species of loyalty (R v O'Connell (1844) 7 ILR 261).
There were four kinds of allegiances (Rittson v Stordy (1855) 3 Sm & G 230; De Geer v Stone (1882) 22 Ch D 243; Isaacson v Durant (1886) 54 LT 684; Gibson, Gavin v Gibson  3 KB 379; Joyce v DPP  AC 347; Collingwood v Pace (1661) O Bridg 410; Lane v Bennett (1836) 1 M & W 70; Lyons Corp v East India Co (1836) 1 Moo PCC 175; Birtwhistle v Vardill (1840) 7 Cl & Fin 895; R v Lopez, R v Sattler (1858) Dears & B 525; Ex p Brown (1864) 5 B & S 280);
(a) Ligeantia naturalis, absoluta, pura et indefinita, and this originally is due by nature and birthright, and is called alta ligeantia, and those that owe this are called subditus natus;
(b) Ligeantia acquisita, not by nature but by acquisition or denization, being called a denizen, or rather denizon, because they are subditus datus;
(c) Ligeantia localis, by operation of law, when a friendly alien enters the country, because so long as they are in the country they are within the Sovereign's protection, therefore they owe the Sovereign a local obedience or allegiance (R v Cowle (1759) 2 Burr 834; Low v Routledge (1865) 1 Ch App 42; Re Johnson, Roberts v Attorney-General  1 Ch 821; Tingley v Muller  2 Ch 144; Rodriguez v Speyer  AC 59; Johnstone v Pedlar  2 AC 262; R v Tucker (1694) Show Parl Cas 186; R v Keyn (1876) 2 Ex D 63; Re Stepney Election Petn, Isaacson v Durant (1886) 17 QBD 54);
(d) A legal obedience, where a particular law requires the taking of an oath of allegiance by subject or alien alike.
Natural allegiance was acquired by birth within the Sovereign's dominions (except for the issue of diplomats or of invading forces or of an alien in enemy occupied territory). The natural allegiance and obedience is an incident inseparable to every subject, for as soon as they are born they owe by birthright allegiance and obedience to the Sovereign (Ex p. Anderson (1861) 3 E & E 487). A natural-born subject owes allegiance wherever they may be, so that where territory is occupied in the course of hostilities by an enemy's force, even if the annexation of the occupied country is proclaimed by the enemy, there can be no change of allegiance during the progress of hostilities on the part of a citizen of the occupied country (R v Vermaak (1900) 21 NLR 204 (South Africa)).
Acquired allegiance was acquired by naturalisation or denization. Denization, or ligeantia acquisita, appears to be threefold (Thomas v Sorrel (1673) 3 Keb 143);
Local allegiance was due by an alien while in the protection of the Crown. All friendly resident aliens incurred all the obligations of subjects (The Angelique (1801) 3 Ch Rob App 7). An alien, coming into a colony also became, temporarily a subject of the Crown, and acquired rights both within and beyond the colony, and these latter rights could not be affected by the laws of that colony (Routledge v Low (1868) LR 3 HL 100; 37 LJ Ch 454; 18 LT 874; 16 WR 1081, HL; Reid v Maxwell (1886) 2 TLR 790; Falcon v Famous Players Film Co  2 KB 474).
A resident alien owed allegiance even when the protection of the Crown was withdrawn owing to the occupation of an enemy, because the absence of the Crown's protection was temporary and involuntary (de Jager v Attorney-Geneneral of Natal  AC 326).
Legal allegiance was due when an alien took an oath of allegiance required for a particular office under the Crown.
By the Naturalization Act 1870, it was made possible for British subjects to renounce their nationality and allegiance, and the ways in which that nationality is lost are defined. So British subjects voluntarily naturalized in a foreign state are deemed aliens from the time of such naturalization, unless, in the case of persons naturalized before the passing of the act, they have declared their desire to remain British subjects within two years from the passing of the act. Persons who from having been born within British territory are British subjects, but who at birth became under the law of any foreign state subjects of such state, and also persons who though born abroad are British subjects by reason of parentage, may by declarations of alienage get rid of British nationality. Emigration to an uncivilized country leaves British nationality unaffected: indeed the right claimed by all states to follow with their authority their subjects so emigrating is one of the usual and recognized means of colonial expansion.