Interstellar Interventions

In how many works of sf must Terrestrial governments respond to a powerful extrasolar spaceship that has entered Earth orbit?

The Byworlder by Poul Anderson
Childhood's End by Arthur C.Clarke
"Skysign" by James Blish

In Anderson's "Soldier From The Stars," I think that the aliens land on an island? - although I am not about to go upstairs to dig out the story. (Later: see here.)

"...the New Hampshire sky was filled with the thousands of starships of the Galactic Milieu, and the Great Intervention had begun."
-Julian May, Intervention (London, 1987), 31, p. 665.

In Clarke's Rendezvous With Rama, a spaceship passes through the Solar System. In many other works, like Brian Aldiss' Equator, the extrasolar aliens are present, either covertly or overtly. Anderson treated this theme several times, usually in short stories, e.g.:

"Details"
"No Truce With Kings"
"Interloper"
The War Of Two Worlds
"Peek! I See You!"
"Backwardness"

There are others. Memories of short story titles recede into the temporal distance.