Matt Creighton means that the reach of the war's destruction is great - the war affects everyone, even those not directly involved in the fighting.
Of the four Creighton boys old enough to join the troops, three of them, John, Tom, and Eb, have joined the Union forces. Introspective Bill, however, in following his own conscience, has gone to fight with the Confederates. Some of the rabble-rousers in Jasper County do not take Bill's action kindly, and consider him a traitor. Previously, Jethro Creighton had been accosted at the store by some of these characters, and when asked if his father condemns Bill's decision, Jethro speaks up in defense of his brother, saying that his father says nothing about it, but that he himself still retains the utmost love and respect for his brother as a person. Angry that Matt Creighton will not speak out against Bill's traitorous actions, these drunken men deliver a threat to the Creighton's home in the dead of night.
Beaten down by the stress brought on by the war, Matt has a stroke and is disabled. When the family receives the sinister warning, he says with despair,
"This war is a beast with long claws".
Not only does the war result in the killing and maiming of thousands on both sides and the destruction of all that lies in its wake, it affects everyone - tearing up families and bringing hardship to those not even directly involved, including the sick like himself, and children like his son Jethro (Chapter 6).