Iraq, Iran and North Korea - referred to by the Clinton administration as "rogue states" - have been relabelled an "axis of evil" by President George W Bush.
President Bush's pointed reference to Iraq in his State of the Union address suggests that he intends to take some kind of action against Baghdad before the end of his presidency.
Military budget: $1.4bn Army: 383,000 Combat aircraft: 200-300 Missiles: Small number of short range surface-to-surface types Weapons of mass destruction: Trying to produce nuclear, biological and chemical weapons
Despite years of weapons inspections by the United Nations and international sanctions, Iraq is suspected of still wishing to pursue programmes to develop nuclear, chemical, and biological weapons, and missile developments.
Analysts suggest the US would need to deploy at least 250,000 troops to seriously threaten Iraq's 383,000-strong army.
Iraqi forces are likely to be more resilient than in the Gulf War if the US objective is the removal of President Saddam Hussein.
Iraqi soldiers are already reported to be digging trenches in preparation, and the country's air defence systems have also been upgraded.
Although moderate elements have emerged in Iran and there are some signs that Washington seeks a reappraisal of relations, deep hostility and suspicion between the two countries remains.
Military budget: $7.5bn Army: 513,000 Combat aircraft: 476 Missiles: About 200 conventional medium range Scud missiles Weapons of mass destruction: Chemical and biological; believed to be developing nuclear weapons
Iran remains on the US State Department list as a state sponsor of terrorism. And Washington is also concerned that Iran has regional ambitions.
The US believes Iran is developing long-range ballistic missiles and weapons of mass destruction and will probably have them by 2015.
President Mohammad Khatami's support among moderates is strong, but hard-liners control the military, intelligence, judiciary and security forces.
Iran also has a strong enough navy to "stem the flow of oil from the Gulf for brief periods," according to US Defence Intelligence Agency Chief Vice Admiral Thomas Wilson.
But, according to a report from the US-based Centre for Strategic and International Studies, while Iran's coventional forces are large, much of its equipment is delapidated and obsolescent.
Washington perceives that the most serious threat from North Korea comes from its long-range ballistic missile programme.
Pyongyang is also major exporter of sensitive ballistic missile technology to states like Iran, Libya, Syria and Egypt.
Military budget: $1.3bn Army: 1,000,000 Combat aircraft: 800+ Missiles: 500 conventional medium range, longer range in development Weapons of mass destruction: Chemical, some biological capacity, developing nuclear weapons
North Korea is projected to have ballistic missiles capable of reaching the United States by 2015, and may have the plutonium to build one or two nuclear weapons.
Pyongyang, however, is complying with an agreement to freeze aspects of its nuclear program, and the country remains beset by a serious famine.
In its final years, the Clinton administration appeared to make progress in attempts to engage North Korea in dialogue.
However 37,000 US troops remain deployed in South Korea to counter the threat from the North's one million strong army, and President Bush seems to have put any hopes of further rapprochement firmly on ice.