"conquer by flood and by field"

Solution 1:

There were several prior uses of the phrase "by flood and by field" and "by flood and field". It means by water and by land.

For example, The Enigmatical entertainer and mathematical associate for the year 1829, No. II at page 56 says:

While he will his “ voyages and travels ” recount , “ By flood and by field " what he's had to surmount

The OED has 2 poetic definitions of the word "flood":

A body of flowing water; a river, stream, usually, a large river

and

In wider sense: Water as opposed to land, often contrasted with field and fire