what does "raining down obstacles upon them from the sky"mean?
To men who were accustomed to dealing with President Kennedy, it was like listening to a foreign tongue. Both in public and in private, Kennedy had been as direct as his pointed finger at televised press conferences. Johnson approached a strongly fortified position by outflanking it, or burrowing under it, or surprising the defenders from the rear, or raining down obstacles upon them from the sky, or starving them into submission. Rarely, and then only reluctantly, would he proceed directly from A to B. To him the shortest distance between two points was a tunnel.

Question

what does "raining  down obstacles upon them from the sky"mean?
To men who were accustomed to dealing with President Kennedy, it was like listening to a foreign tongue. Both in public and in private, Kennedy had been as direct as his pointed finger at televised press conferences. Johnson approached a strongly fortified position by outflanking it, or burrowing under it, or surprising the defenders from the rear, or raining down obstacles upon them from the sky, or starving them into submission. Rarely, and then only reluctantly, would he proceed directly from A to B. To him the shortest distance between two points was a tunnel.

Answer

An obstacle is something that is in someones way (stopping them or delaying them), it may be a worry or a problem or it may be something physical like a wall or perhaps a fallen tree across a road, these all represent an obstacle.....
When we talk about something raining down on us, it means a lot of something falling on us at once ( like when it pours with rain).... If we put these two together this sentence means that a lot of obstacles (in one form or another) are wanted to stop who ever or what ever or to get in the way of something/someone.

Answer

When rain fall, it usually falls from the sky. You did know that, right?
When water rains down on you, you get wet.
Now use this image to understand your sentence.

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