Why could the first half of the nineteenth century be characterized as one of momentous changes ?

It can be characterized as one of momentous changes because of the industrial revolution in America occurs within that time period.

Explanation:

Industrial revolution brought in modern inventions that gradually replace the way Americans usually conducted our economic activity.

In economic perspective, this brought in massive income growth due to the additional outputs we're able to produce. Making us able to accumulate more wealth and slowly establishing yourself as one of the strongest nations in the world.

But , the effect of this industrialized revolution does not impact our economy alone.

It also affect our social situations and culture. Before the industrialization happen, united states relied heavily on slaves as our main labor.

Since industrialization come on northern states first before it can spread to the southern states, it creates a huge cracks within our nations regarding our position on slavery (the northern states wanted to abolish it, while the southern states want to keep it.) This disagreements lead to the civil war between April 12, 1861 – April 9, 1865  that re-shape the way we view our own society as a whole.

Because of these huge impacts, we can state that first half of the nineteenth century be characterized as one of momentous change

the correct answer is: the fact that the anti-federalist papers were considered poorly written.