what has "become necessary"? what does this phrase imply about the actions taken by colonist prior to the declaration?

Why did the federal government choose to give each tribal member a 25-year deed to the reservation land? this was the life expectancy of native americans at this time, and the government wished to grant rights for only a single generation. they considered this to be enough time to keep the area calm before they could take the land back again for sale. the united states government believed that the nutrients from the land would be depleted by the end of the deed. the government believed this would be enough time for the tribal members to become self-reliant and adapt to the farming lifestyle.