Why was the land war more difficult for the united states than the war at sea?

I'm not sure what era of wars you're referring to, but I can give you a fairly general answer.
War on land tends to be more difficult due to the terrain. There are mountains and valleys, soldiers have to walk great distances to get to their destination if their vehicles cannot handle the landscape, and there's always the possibility of guerrilla warfare (hiding in the bushes, attacking and running away) from the other army.
While war at sea can be treacherous, it also lends itself to finding the target easier. Water tends to try to remain level, so there aren't any hills or valleys to contend with, and a telescope can see for miles on the open sea.
If you can give a specific war you're referring to, I could try and give you a more specific answer. 

Accumulating more land so that your country can continue expanding without running into problems such as overpopulation

productive most commonly means: achieving or producing a significant amount or result.

explanation:

productive can also mean:

producing or giving rise torelating to or engaged in the production of goods, crops, or other commodities.

these depend on how the word productive is used. most likely productive will mean   achieving or producing a significant amount or result.