score:10
The war with Great Britain was justified because when Hitler invaded Poland, Great Britain declared war. The invasion of Poland itself was justified by the "provocation" at Dantzig.
For specific operation "Sealion" and Battle of Britain, there were more justifications given to the public, especially German population:
Upvote:2
Technically, given that Britain had declared war on them and then rejected terms, Germany didn't have to give any reasons.
As for persuading the people to keep on fighting, they touted their prior victories, and derided British attempts to keep up morale by pointing out all the prior German victories had not actually subjected Britain. Furthermore, the anti-British propaganda got significantly more nasty in tone. The British in Nazi propaganda were plutocrats ("the Jew among the Aryans"), oppressors of their empire, and hypocrites. Der Fuchs von Glenarvon and My life for Ireland depicted them as oppressors of the Irish. Ohm KrΓΌger depicted the Boer War as the horrors they inflicted on the Boers, in particular playing up the concentration camps.
Upvote:10
You only need a public justification to start a war. Or to continue a war after the other side offers a peace deal.
Nazi Germany had not (directly) started the war with Britain. Britain had declared war. And they were not offering peace either.
So the justifiction was easy: "Britain is an enemy that is attacking us. And they won't stop until we make them."
Granted, after the victory over France and with only Britain left in the war, it did not need much convincing either.