Ukraine and Russia have technically been at war since 2014 but neither has declared war publicly.
What's the reason not to declare war if it even mentioned in the Wikipedia?
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I think the main reason is probably because, if it's an "irregular war," other nations with interests, agreements or obligations with either or both parties can pretend nothing, really, is going on.
If a formal, internationally official war is declared, then nations and groups of nations would be more likely to act, either on their own, or by being pushed into it by formal agreements, conventions or obligations.
Not declaring war gives both parties an "out" to back down or de-escalate if things are looking like they are moving in a direction they don't like. An actual war gives less options.
The United States has pretty much been in a constant state of war for the past 70 years or so, including a pre-emptive invasion, conquering and occupation of Iraq, and yet, they haven't been "officially" at war since WWII.
I'm not sure that a formal declaration has much meaning in a modern context.
From the Ukraine side:
From the Russian side:
The short answer is that the practice of declaring war is no longer regarded as necessary; and as the other answers mention, it may provoke the other party, or force other countries to get involved.
Wikipedia's article on declaring war states:
Since 1945, developments in international law such as the United Nations Charter, which prohibits both the threat and the use of force in international conflicts, have made declarations of war largely obsolete in international relations.
And indeed, the list in that article of declarations of war since 1945 is very short - just 15, of which 4 are still in force (Syria, Iraq & Lebanon vs Israel (1948); Egypt vs Islamic State (2015)).
Notable among the absences from that list are the Korean War and Vietnam War - and the conflict in Ukraine.