US Government:

Department of Homeland Security: Primary responsibility is to restore power and communications. Accomplishing this, DHS sent out a reverse 911 call (apparently originating from Topeka, in Jericho's case)urging everyone to remain calm and stay where they are.

FEMA: Primary responsibility is to assist in evacuation of areas most affected by the attacks and provide humanitarian aid where possible. Last known to have evacuated the city of Rogue River, KS, due to contamination of the town's water supply by the attack on Lawrence, KS.

Private Security Forces: Due to the overstretched nature of the United States Armed Forces, DHS is forced to hire what are essentially mercenary forces, independent of the government. At first, one group, Ravenwood, is tasked to protect the Governor of Nebraska. However, this group is then ordered to Rogue River, KS, to assist in evacuation efforts. While there, the group stops taking orders and begins shooting all hospital patients either too sick or wounded to be evacuated. After this, the group moves towards Jericho, using their previous orders as cause to take whatever supplies they need from the town. They are, however, turned back, though they vow to return.

Armed Forces:
Stanley Richmond sees a line of five tanks headed west on Interstate 70, towards Denver. Flight recorder tapes from a downed airplane also indicate that fighter jets are dispatched in the same direction shortly after the bombs go off.

Five days after the attacks, two ICBM's are seen flying eastward, seconds before an electromagnetic pulse knocks the power out again. Their destination remains a mystery (but the shows producers have said that the missiles did hit their intended targets).

Because we have not seen hide nor hull of them, we can assume that the US Military, or the various factions of it, are either busy shooting it out with each other at the whims of the 6 presidential contenders, or are indeed conducting relief and just being slow.

International Response

China: Assumed by some of the residents of Jericho to be behind the attacks, based on their inability to translate spoken Chinese during a brief broadcast received on the television at Bailey's Tavern. China, however, apparently responds to the attacks with humanitarian aid, which it airdrops from Russian-made cargo planes (China owns many Soviet-era planes). Despite this show of friendship, the residents remain suspect, as each pallet of food or supplies contains a printed flier stating "DO NOT FIGHT: CHINA IS YOUR FRIEND," leading some to believe an invasion force is on the way.

Germany: In the episode "Black Jack," Heather's friend states that they received a airdrop from Germany.