One of Two

I have taken this question from the position of the development of the history of the political philosophy and international legal notion of declaring war. As such it is only 'one' possible answer.

The notion of the Declaration of War sits currently in the Jus ad Bellum – the ‘right to war’. This is a set of semi-legal, semi-moral criteria developed over many centuries that seeks to regulate how wars are conducted. Jus ad Bellum governs both when a state is ‘allowed’ to wage war but also how a state should start a war. (How a war is conducted once began is a separate related theory - Jus in Bello) Jus ad Bellum is a cornerstone of what is called the ‘Just War’ theory.

Rome never fought an unjust war

Turning to history, as the name might have suggested, Jus ad Bellum is a concept that goes at least to Ancient Rome, and it is the Roman conception that most informs at least the extended European civilisation’s concept of war. The Roman historian Livy describes the custom of declaring war as being a religious one carried out by priests in the name of Jupiter. If the rites, including declaration, were not done, then the war could not be just. Following this, in one conception, all Roman Wars were defensive wars. It followed: the enemy had aggressed against Rome in some regard – had breached the ’Law of Nations’ as Seneca describes it -, the Romans in response demanded reparations/retribution, and if/when that wasn’t forthcoming Rome declared it’s now justified war. The 'purpose' of such a declaration was to appease the Gods. If you're at war, the last thing you want is your gods being offended because you didn't declare the war properly. In the later empire the process was semi-secularised and legalised, the priest replaced with Legate which became convenient in a Christianising empire that no longer cared about Jupiter but still cared (to some degree) about the Mos Maiorum the ‘ancestral customs’ that defined being a good Roman, and included such ancient rites as declaring war.

The Father of International Law

There are some fascinating developments in the middle-ages on the notion of Just War especially from St Augustine and St Thomas Aquinas, as well as Holy War, and the (questionable) right of the nobility to wage private war. There is also the fascinating theories of the Dar al-Islam, Dar al-sulh and Dar al-harb in the Muslim world, which I'm not sufficiently qualified to go into.

But for our purposes, I’m going to skip to the 17th century and Hugo Grotius, the ‘father of international law’. Grotius was a diplomat, lawyer and theorist working at various times for the Dutch, the French and the Swedish. Grotius had several things to say about the virtues of War Declarations.

First, Grotius found that declarations distinguished soldiers from bandits and pirates e.g. Sir Francis Drake was sometimes a Pirate and sometimes a Privateer depending on whether a declared war was ongoing at the time.

Second, Grotius discusses that war is a public not a private affair. The only legitimate way Grotius argues a government may bind its citizens is through law. As such for a Nation to become a Nation at Arms the Government must engage in the war legally through a declaration.

Third, Grotius makes a confused point about whether a declaration gives nations a last opportunity of diplomacy between the two nations from which war may be averted.

Grotius also makes points on whether a declaration might be required to ‘seize one’s own property’ (Yes), the source of power to declare war (The Sovereign), and on the difference between conditional declarations (e.g. 1939 Britain against Germany if Germany didn’t withdraw from Poland) and absolute declarations (e.g. USA on Japan in 1941).

Importantly, Grotius ties his theory to a Natural Law, that there is a universal right way for states to act, and that this right way may be 'discovered' by theorist. This is a moral proposition, but one that has some perhaps surprising endurance in different guises until today. Grotius was very much in line with the early Enlightenment that had begun. Grotius is important as he was working in a whole school of the rules of nations and the rules of war and peace. His contemporaries Emer de Vattel, Samuel von Puffendorff, and Emmanuel Kant worked in the same schools and on the same issue. The important point is that the political theory of war and peace was re-emerging, in a more legally coherent and legally internationalised world. The same basis theories continue to be important today.

Guerrillas and Policemen: A breakdown of formality

Through the middle ages and early modern period, war became increasingly a formal matter. Partly this was through the work of Grotius and others (for Example, Grotius worked for a while as a lawyer in the Dutch Prize Court, where spoils of war - like captured warships - were litigated). Elsewhere, increasingly professionalised and meticulously regimented armies marched in unison across Europe. Many wars were declared, though some were not (e.g. the Quasi War). Alongside this, declaration of war became tied into the notions of the Sovereign and Monarchy. Peace also began to acquire elaborate negotiations and guarantees. The legalisation of war in a modern sense had begun. Declarations were a matter of prestige and honour for the Courts, especially as alliances began to both be formalised and shift frequently. To act dishonourably against a state in one war by not formally declaring could cost you that state as an ally in a future war.

However, as war became ‘regular’, the irregular war began to emerge. In a time of systematic upheaval due to the French Revolutionary Wars, States began operating in less than upfront ways. Spain fought the ‘guerrilla’ the little war again Napoleon’s occupation. The Spanish King had capitulated but armed bands opposing the Bonaparte’s rule over Spain began to harass the French. This was in breach of such concepts of Jus in Bello and Jus ad Bellum and might be seen in theory as an undeclared continuation of the war (it also exhibits that the ongoing development of Jus in Bello and Jus ad Bellum are intertwined). True to what Grotius had theorised, there was much lawlessness of the guerrillas and sometimes little to distinguish them from bandits before they were ultimately formalised into the Spanish Army again. Victory erases many sins, and although the legality of the Guerrilla War was questionable, their effectiveness was not doubted. If such undeclared war could work in Spain, it could perhaps work elsewhere.

The ‘Policeman of Europe’ was a catchy name for a repressive reality. Following Napoleon’s defeat, a compact was made in Europe. Revolution was not to be tolerated. The Russian Empire emerged as perhaps the most conservative, reactionary and anti-revolutionary state in Europe, and its Tsar Nicholas I became the eponymous Policeman of Europe. For our purposes we must look at the 1848 revolt of the Hungarians. With their Habsburg rulers unable to control the revolt, 300,000 Russia soldiers. This was a massacre, but it was not a declared war. So to, a hundred years later, when Hitler ‘restored order in Austria’ during Anschluss, it wasn’t an 'invasion' or so the Nazi claim went, it was a restoration of order. And so when President Harry Truman went into the Korean he told Congress it was a 'police action' and was expressly not a war. Careful words to disguise realities. The point being, that States were relearning what they had always really known: an undeclared war muddies the waters of legitimacy and legality, and it can sometimes be effective at pursuing an objective.