Amos 3: the chosen people of the Lord will face consequences of their idolatry

Amos argues that something happens in response to another action. For instance:

3-7
Do two people walk hand in hand
    if they aren’t going to the same place?
Does a lion roar in the forest
    if there’s no carcass to devour?
Does a young lion growl with pleasure
    if he hasn’t caught his supper?
Does a bird fall to the ground
    if it hasn’t been hit with a stone?
Does a trap spring shut
    if nothing trips it? (...)

In particular we read:

(...)-7 And when disaster strikes the city,
    doesn’t God stand behind it?
The fact is, God, the Master, does nothing
    without first telling his prophets the whole story.

Amos is a prophet. Israel/Samaria is doing economically fine - it seem that there is nothing to worry; however, the prophet Amos is warning Israel about a disaster that the Lord will send to them.

9-11
Announce to the forts of Assyria,
    announce to the forts of Egypt—
Tell them, “Gather on the Samaritan mountains, take a good, hard look:
    what a snake pit of brutality and terror!
They can’t—or won’t—do one thing right.” God said so.
    “They stockpile violence and blight.
Therefore”—this is God’s Word—“an enemy will surround the country.
    He’ll strip you of your power and plunder your forts.”

The Lord is calling pagan countries to witness how the Lord will deal with his people (that can't and won't do anything right). An enemy (Assyria) will strip Israel of his power.    
The Lord wants that his people behaves accordingly to their calling:

2
“Out of all the families on earth,
    I picked you.
Therefore, because of your special calling,
    I’m holding you responsible for all your sins.”

This calling is to love the Lord and not false gods:

13-15
“Listen and bring witness against Jacob’s family”—
    this is God’s Word, God-of-the-Angel-Armies!
“Note well! The day I make Israel pay for its sins,
    pay for the sin-altars of worship at Bethel,
The horned altars will all be dehorned
    and scattered around.
I’ll tear down the winter palace,
    smash the summer palace—all your fancy buildings.
The luxury homes will be demolished,
    all those pretentious houses.”
        God’s Decree.