How does the mood of the poem "There Will Come Soft Rains" change dramatically?

Mood is the emotion caused by reading the poem.  A poem might be sad, happy, excited, passionate, or mourning, just like a person.  Any mood that a person has can be found in poetry. This little poem is very interesting, because it is about the effect of war on nature.  The poem starts out with a very serene mood, and ends up with a depressingly warning tone that makes the reader thoughtful. 

The beginning of the poem starts off in a way that seems very mellow and inviting.  It takes us through nature, and it is raining, but softly.  Everything is calm and sweet. 

There will come soft rains and the smell of the ground, 

And swallows circling with their shimmering sound; 

And frogs in the pools singing at night, 

And wild plum trees in tremulous white; 

Robins will wear their feathery fire …

The beginning of the poem may be calm and celebratory in nature, but there is a shift in the middle.  Now the poem is talking about war.  There is a war on, and humans are destroying ourselves.  The poem may make us surprised at this point, because we were talking about frogs and plum trees a moment ago, and now we are talking about war.

The ending of the poem is very dark.  The mood is cautionary.  If we do not change our ways, this is what will result for us.  We will destroy ourselves, and nature will not care.

Not one would mind, neither bird nor tree

If mankind perished utterly;

And Spring herself, when she woke at dawn,

Would scarcely know that we were gone.

The implication is that nature and animals were here before our civilization and will be here after.  We can war with each other and destroy each other, and nature will heal itself and go on.  Nature is not interested in the least.  This may make the reader feel depressed, but in a way the poem is also reassuring.  We may destroy ourselves, but nature will be fine.