"A nation formed through a revolution needs to recognize that rebellion is an essential aspect of its character and its destiny. Jefferson's letter to Madison continued: "Unsuccessful rebellions indeed generally establish the encroachments on the rights of the people which have produced them. An observation of this truth should render honest republican governors so mild in their punishment of rebellions, as not to discourage them too much. It is a medicine necessary for the sound health of government." (Thomas Jefferson, “Letters of Thomas Jefferson.”)

What does the emphasized part mean?

"Encroaching" is the insidious, almost imperceptible taking over of something.

There's a lot of talk currently of how the ocean has encroached upon countless number of islands all over the world's seas, some of which have completely disappeared.

It has probably been doing it all along, but because the process is so slow, so stealthy, so quietly done, that it hasn't been noticeable. This current encroachment is happening at such a rapid rate that it's catastrophic'

So, the sense of what he's saying is that changes for the worse happen so imperceptibly, so quietly, that the changes they introduce into a society soon become the norm.

Neither is it a tsunami or a flood which is causing whole islands full of people to abandon their homes and habitats. It's the constant drip drip drip of an iceberg melting drop by drop into the sea far away.

It doesn't need to be a change in government, or a war, which brings about negative change in society/governments. There's always unscrupulous people in every society, and within all parts of society. But they are usually unable to dominate because Society and Government work together against this trend.

IF, however, a society has gradually, without even realising it, accepted practices which are against the good of the society/community, they've already already taken a step down the wrong path. If things are done constantly, or heard constantly they soon become normalised. And once one form of questionable behaviour becomes 'normal' the next step further down that wrong path is easier until the whole of society is corrupted.

So what he's saying in your marked bit is that: Whether they succeed or not, rebellions generally have effects in their aftermath. Successive governments, instead merely of addressing the factors which directly caused the rebellion: should, he insists, examine ALL the steps that took them further and further down the wrong path right back to the very first one. The metaphorical fork in the road.

(I went into more detail here because it's really important to understand what he's saying here.)

Thank you, friends.