The Vietnam War, for example, was attacked as unconstitutional because there was no formal declaration of war.

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Sudan has made no formal declaration of war, but analysts say Mr Kiir is clearly escalating the war of words.

BBC: South Sudan's Salva Kiir says Sudan has declared war

The problem here was that it was not clear that had he asked for a formal declaration of war he would have gotten one.

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It has announced a formal declaration of war on the South, and pledged to reopen a mothballed nuclear reactor in defiance of UN Security Council resolutions.

BBC: North Korea warns foreign embassies to prepare escape

North Korea has also announced a formal declaration of war on the South, and pledged to reopen a mothballed nuclear reactor in defiance of UN Security Council resolutions.

BBC: 'No plans to withdraw' UK diplomats from North Korea