Why so many nations declared war on Germany in 1945? I mean this wasn’t beneficial to them, so why?
It actually was beneficial because it would have guaranteed a seat at the post-war negotiations.
Good publicity mostly: “look, we don’t like those nasty Nazis either!”
I think Turkey, in particular, was bribed with aid and informal offers of protection from now ascendant the Soviet Union. Finland needed to declare war to make sure the USSR didn’t gobble them up and being an ally made them immune. As for the South American countries, economic pressure and economic concessions which they lost NOTHING by signing on.
Well… Finland started fighting against the Germans in September 1944. But it did not technically declare war on Germany at all - the Finnish Parliament saw in March 3rd 1945 that there had existed a state of war between Finland and Germany since September 1944 - so it was conveniently backdated. Coincidentally no one bothered to end that state of war either. For a while. In 1950s someone found that out. So the war between Finland and Germany officially ended on March 12th 1954 (fighting had ended on April 27th 1945…). Technically this wasn’t all that problematic since neither the DDR or the BRD was the continuator state of Nazi Germany.