'War' (noun) can be both countable and uncountable. As an uncountable noun, it means 'armed fighting (usually between nations)'. For example, 'the two countries have been at war for years'. We can add detail about the kind of war by adding another noun, or an adjective, in front: civil war, cold war, nuclear war, gang war etc.
As a countable noun, 'war' means, 'an example or period of armed fighting (usually between nations)'. For example: the American war of independence, war graves, war poetry etc.
Antonio, you ask the important question, 'how can someone see the future in the war?' This is a question that demands serious answers, not error correction!
Anyway.......you use the uncountable meaning of the noun 'war' in your question. Because 'war' in your question is uncountable, and you are talking about war in general, you don't need to use an article. When we are talking about things, or people, in general, we don't usually use 'the' with uncountable (or plural) nouns. Another example, from the John Lennon song 'Imagine' that you quote is, 'living life in peace', not 'living the life in peace'.
Another change I'd like to suggest to your sentence, is using 'anyone' instead of 'someone'. Usually, but not always, 'some(one)' is used in statements and 'any(one)' in negatives and questions. We can use 'some' in questions when we expect or hope that someone will say 'yes'. For example, 'would you like some more to eat?'.
'Someone' usually means we are thinking of only one person, 'there's someone on the phone for you'. Compare, 'did anyone phone while I was out?'. 'Anyone' has an open, non-specific meaning and usually suggests an indefinite number or amount.
I think your important question needs to be, 'how can anyone see any future in war?'.
See you tomorrow.
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