Meaning of 'but' in these Kipling's lines
Please don't mind my questions if they seem elementary. Obviously I was brave enough to attempt some classic poetry and the troubles were inevitable... I checked out the available dictionaries and it did not helped much, so I am kindly asking for your help.

1st instance:

* ' And two have come back to Fort Bukloh where there went forth but one.'

Does 'but one' mean here 'this time not by one'?

Is 'there went' used to describe an actual movement, or does it, like 'there goes', mean that the object has appeared/vanished?

2nd instance:

* 'What is the flag of England? Ye have but my bergs to dare, ye have but my drifts to conquer. Go forth...'

Is 'but' equal to 'only' here? 'You only have to conquer my drifts'?

Question

Meaning of 'but' in these Kipling's lines
Please don't mind my questions if they seem elementary. Obviously I was brave enough to attempt some classic poetry and the troubles were inevitable... I checked out the available dictionaries and it did not helped much, so I am kindly asking for your help.

1st instance:

*  ' And two have come back to Fort Bukloh where there went forth but one.'

Does 'but one' mean here 'this time not by one'?

Is 'there went' used to describe an actual movement, or does it, like 'there goes', mean that the object has appeared/vanished?

2nd instance:

*  'What is the flag of England? Ye have but my bergs to dare, ye have but my drifts to conquer. Go forth...'

Is 'but' equal to 'only' here? 'You only have to conquer my drifts'?

Answer

1: 'but one' here means 'only one'. 
I think that you have broken it up incorrectly. the words that go together there are 'went forth' - which means 'left'. So the whole thing means 'two (people) came back to the Fort, even though only one left'
2: Yes, again it is 'only'.

Answer

In BOTH sentences, the word "but" is used to mean only. You can think of it as a shortened way of saying "nothing but". Look at these examples:

"You ate nothing but toast for breakfast today."
"Ye ate but toast for breakfast this morning."

In the first sentence to "go forth" is the opposite of "come back". So the sentence says that only one left (went forth), but two have returned (come back).

I hope this helps. Kipling can be tough even for native speakers at times, so I applaud your effort.