If aggressive war is always wrong, is defensive war always right?
No, how can it always be right? Suppose a nation armed only with five pea shooters declares war on another country, which in return defensively nukes the aggressor nation, leaving not a single survivor- would you consider that to be justified? All kinds of factors need to be taken into account, not the least being the proportionality of the 'defensive' measures.
Just War theory is a well established area of philosophy. See eg. Just War Theory (IEP), War (SEP), Just War Theory (Wikipedia). The way you've framed your question seems to just be asking for opinions on the options.
War is a fact of human society, and almost invariably injustice, or at least perceived injustice, is at the root of why. You can't legislate war out of existence, any more than really good moral theories can make people stop doing what they know to be immoral. I think this is important to bear in mind.
The existence of and consequences to using nuclear weapons, has led to the attempt to create a 'rules based international order', policed through sanctiins and negotiation wherever possible, rather than by force. See eg Is there such a thing as a rules-based international order? (Financial Times interview transcript). Specific periods of history drove the creation of laws and penalties for genocide, and the defining of certain weapons and methods as war crimes. So how we think about and respond to war changes over time. We are on the brink of handing a lot of the frontline over to drones, and there's a very real risk they will be increasingly autonomous, and controlled by AI. Biological weapons and nanomachines are poised to be able to do ever more dangerous things.
Large scale imprisonment as a penalty, rather than execution, enslavement or massacre, only became an alternative after the agricultural revolution. Before that food was just considered too expensive for imprisonment of those who couldn't pay for their own food. In the Bronze Age this was even more extreme, with child sacrifice to Tophet in Carthage if the parents couldn't pay taxes, or human sacrifice in general if a bad harvest threatened too little to eat.
War, and the preperation and conduct of it, are part of social and technological life, so treating the morality of war isolated from that can't work. Society and ways of life, change. And so does what war is.
You might like to have a look at war as defence of the social contract: Exploring philosophy behind "Catch-22" novel: individual in war
War and revolution related to game theory: Is the tyrannicide perpetrated by William Tell morally legitimate?
The hidden costs of eliminating human tendency towards or capacity for conflict: If we had the ability to make humanity less war-like, should we? It's worth saying there's good evidence highly economically unequal societies have more violence because of the limitations on avenues by many to gain status - every threat to status then becomes existential. In more equal societies, everyone has access to sports, arts, to take up new careers. We can picture something similar for war, that very often it's used as a distraction, or to cover over fractures in a society by unifying towards a threat. Positive alternatives are possible, like organising an Olympic Games, which both in Ancient Greece and now, provide a kind of substitute for the logistics and training and competative spirit of war.
Positives of revenge: What are the arguments for revenge and retribution?
If individuals are legally allowed to defend themselves (with proportionality) when aggressed by other individuals, then groups of people are allowed to defend themselves (with proportionality) against other groups. That would include nations, so defensive wars are legal in international law.
IOW, nobody can reproach you anything if you just defend yourself.
Does it make it moral? Not necessarily, as one could argue that a non-violent or submissive response is morally or even strategically superior in some particular cases, eg in case of a weak-to-strong situation.
What for, for a declaration of war? Perhaps a non trivial defence of national sovereignty would be sufficient. You might want to read Clausewitz
War is the continuation of policy with other means.
Then defence of national sovereignty is policy (which seems about right), but if we add 'all policy', then every non-trivial policy may be sufficient to declare war (which seems wrong).
FWIW I don't agree with the claim, even if we may declare war to safeguard the population.
The rules of war are contained in the Geneva and Hague Conventions. Some nations want to make waging aggressive was a war crime in its own right. Currently, the right to defend national sovereignty is unchallenged. This includes the right to defend citizens. Any action must be proportionate. If a nation is invaded, and eventually the invader sues for peace, is the defender bound to comply in all circumstances? If the answer is no, then a stalemated war might continue indefinitely. Are the additional deaths justified to protect sovereign integrity? This is a legal and ethical issue. I am not asking for opinion or discussion of specific cases. Fundamentally, is nationalism ethical?
The question asks for a moral assessment. In addition I want to draw the attention to the basic juridical guide: The law of war, a component of international law. The international law also deals with the question of collective self-defense.
Concerning international war the UNO charta states in article 2:
- All Members shall settle their international disputes by peaceful means in such a manner that international peace and
justice, are not endangered.
- All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or
independence of any state, or in any other manner inconsistent with
the Purposes of the United Nations.
- All Members shall settle their international disputes by peaceful means in such a manner that international peace and security, and
Each international war breaks this rule.
Hence any admissible international war has to be considered an exception from the rule above. The main exception is governed by article 51 of the UNO charta:
Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.
For a list of the present UNO members see https://en.wikipedia.org/wiki/Member_states_of_the_United_Nations.