Explain what claudius says that confirms what Horatio told the sentries earlier about Denmark

In an attempt to avoid battle, King Claudius sends two messengers to the king of Norway, letting him know that his nephew, Fortenbras, was putting together an army to wage war on Denmark and reclaim lands previously lost during the rule of King Fortenbras, his father. This is the same information that Horatio interprets from the visitation of the ghost of King Hamlet, and what he tells the sentries.