My ears pricked up when I heard the local weatherman say that rain was expected to “invade into the River Valley.”
Why, I wondered, hadn’t he said that rain was expected to “invade the River Valley? The verb [b]invade[/b] includes the sense of “into.”
[blockquote][b]invade[/b]:  transitive verb. to enter in a hostile manner, or with armed force; to make an inroad or hostile incursion into.
[/blockquote]The word derives from Latin [i]invadere[/i] “go into, fall upon”
As a transitive verb, [b]invade[/b] takes a direct object:
[blockquote][There was a] French plan to invade Britain in the 18th century
Stink bugs expected to invade W. Va. homes this fall
[/blockquote]Invade may be used intransitively:
[blockquote]If they [aliens] invaded I think that they would pacify every part of the planet …
What’ll we do when they invade?
[/blockquote]The OED lists a construction that uses [i]on, upon[/i], or [i]into[/i] after [i]invade[/i], but doesn’t illustrate the use more recently than 1814.
The construction [i]to invade upon someone’s privacy[/i] has the familiar ring of custom, but the construction “invade into” sounds like careless writing.
Here are some examples of the “invade into” construction in which the sentences would be made stronger by dropping the “into.”
[blockquote]Brazilian pepper constantly trying to invade into Corkscrew Swamp Sanctuary
[Should] media invade into celebrities private life or not?
Why can’t Canada sometime try to invade into the USA?
You will learn basic techniques on how to invade into others’ dreams
When computers started to invade into the field of training,
[/blockquote]This use of the unnecessary “into” seems to be especially common in medical writing:
[i]individual malignant cells can invade into the stroma[/i]
Per cell, more mites invade into shorter and narrower cells
Tumors arising from adjacent organs can also invade into the bladder
the bacterial cells that adhere to and invade into cancer cells. …
cancer occurs when a tumor has the potential to invade into a different tissue
In each of these examples, the word [b]intrude[/b] would seem to be a better choice, and one that works comfortably with “into.”
I suppose that a writer might see some stylistic reason for adding the “into” after [i]invade[/i], but in most cases, [i]invade[/i] is all you need.

I thought there would be discussion followed, but only to find nothing! - -

very professional, i agree with you, "invade" is good enough. you do not need to add "into"

e.g. America invaded Iraq not America invaded into Iraq

It depends on the objective follow. It's been long since I listened/read the explanation, but basically, you use "into" to describe an active movement: "She jumps into the river" - the whole body of "she" completely immerse in the river's water. I would say ignore the media (weather broadcast, news, etc.) but most of the time, scientific papers don't make mistake like that, particularly the medical ones. "Invade" describe the aim, I suppose, while "into" is more of a physical conversion from outside to inside.