The simple past tense conveys the idea that the action described was completed in the past; it is over and done with. With the simple past, there is no expression of a relationship with, or bearing upon, the speaker's sense of The Now.
The present perfect expresses some relationship between the past event or action and the speaker's present. "Edison has invented the light bulb" conveys the idea that the speaker expects the stated fact to be news to the listener, as it was news to him, or that the statement has some other bearing upon "the present day", broadly conceived. Consider an imaginary conversation in 1879:
Dad, will we be able to read at night without kerosene lamps, gas lamps, or
candles in the future?
--Yes, we will, because Edison has invented the light bulb.
In other words: Edison's invention is one you must bear in mind, son, when theorizing about future night-time reading.
If we use a specific time expression that excludes the present, either by confining the action to the past or confining it to the future, then the present perfect will express an idea that does not jibe with that time reference.
I have seen him yesterday. not OK
I have seen him tomorrow. not OK
I have seen him on March 3rd, 2015. not OK
I have seen him as recently as March 3rd, 2015. OK
I have seen him now and then. OK
I have seen him often. OK
I have seen him rarely. OK
I have seen him today. OK, because it means "earlier today" but saw is also possible
Note the "ever" and the indefinite article "a" in the song lyric:
I wanna know, have you ever seen the rain
Comin' down on a sunny day?
Those time references "ever" and "a sunny day" are general, not specific. Ever means "at any time" and today could be a sunny day. These references do not exclude the present.
P.S. On the Korean War example in the OP.
The Korean War has broken out in 1950. not OK
Let's say a newscaster is reporting the news that the war has begun:
War has broken out on the Korean peninsula.
War has broken out in the past twenty-four hours on the Korean peninsula.
War has broken out in the last few days on the Korean peninsula.
Now let's say that the armistice is being signed.
The war in Korea has ended.
Now let's say soldiers up in the mountains, cut off from communications, are still holed up a week after the armistice. A platoon arrives on the scene and informs them that the war is now over.
The war has ended.
"When?" they ask.
The war ended a week ago.
The war's end has bearing upon these soldiers now, to be sure. That idea is expressed in the first sentence even though it lacks any time reference but the tense. In the second sentence, however, we are no longer talking about the war's relevance to the present. The time reference a week ago expressly excludes the present. It speaks of the end of the war itself, not about the end of the war as it relates to the mistaken perception of those soldiers, namely that it was still going on.