You got confused.
My point is that the only way to fight war is to destroy everything. Nothing should be left standing. Thats why USA was successful in WW2 and thats why they failed in Iraq/Afg.
Every war have different dynamics.
Axis powers were responsible for so much death and destruction in various regions that it was easy for the opposing
Allied powers to adopt equally brutal measures against them to rollback their gains and influence public opinion towards this end. WW2 was holistically so brutal that a nation could use a nuclear weapon or two against enemies [in those days] and get away with it without worrying about global reaction.
Operation Enduring Freedom (Afghanistan) and
Operation Iraqi Freedom (Iraq) were conducted under the official narrative of war against terrorism and regimes that supported terrorism. However, Ba'ath Party (led by Saddam Hussein) and Taliban (led by Mullah Mohammad Omar) did not resemble
Axis powers by any stretch of imagination because they were not attacking and destroying other nations. And entire world knew that both regimes faced opposition at home.
Taliban have been targeted for enabling
Al-Qaeda to use Afghanistan as a staging ground for terrorist activities [local and global], therefore
Operation Enduring Freedom had some legitimacy after 9/11 event. However, much of the world knew that Saddam Hussein had nothing to do with terrorist organizations and perceived
Operation Iraqi Freedom as a "neocon agenda." By all accounts,
Operation Iraqi Freedom was an ill-advised venture because important resources were diverted from ongoing
Operation Enduring Freedom towards it and Afghan front suffered from lack of sufficient commitment as a consequence. US had a golden opportunity to (strongly) focus on problems in Afghanistan but neglected it once again.
Now, terrorist groups such as Al-Qaeda and ISIS are certainly perceived as a threat to global security but why would an entire nation be indiscriminately bombed to stop them? These groups are
multi-ethnic and
ideologically-driven and have legs in different nations. They are (not) strictly regional threats that can be contained and/or destroyed by targeting a single (host) nation.
Taliban is also a
multi-ethnic and
ideologically-driven movement and it has legs in both Afghanistan and Pakistan. This is why US military forces conducted offensive operations in both nations, often violating Pakistan's sovereignty in the process (
Operation Neptune Spear is a good example).
US military forces utterly dismantled Ba'ath Party [and its security apparatus] in Iraq because it was a
regional power. However,
multi-ethnic and
ideologically-driven movements such as Taliban, Al-Qaeda and ISIS continue to exist in some form even after loosing thousands of men in various battles because they (somehow) manage to get new recruits from different parts of the world and some nations often fund them and use them as tools of their national interests in the region to increase their influence.
I can explain in great detail how insurgency in Iraq emerged as a well-coordinated and powerful movement after destruction of its security apparatus. Iraq descended into a brutal
civil war in the aftermath of fall of Saddam Hussein due to ethnic tensions between Shia, Sunni and Kurd factions in the nation and foreign intervention to strengthen a favored faction to enable it to achieve supremacy within the country. US was literally caught off-guard in this mess and played a vital role in preventing disintegration of the nation, but at a high cost in men and material. However, lengthy occupation became an expensive venture and President Barack Obama decided to withdraw US troops from the region. US have also experienced a fact that Shia and Sunni factions do not get along well in the Middle East and have decided to stay away from this centuries old ideological conflict. US, at maximum, will defend GCC states in the hour of need from direct aggression from Iran and its allies in the Middle East.
Now, can US utterly obliterate a nation with its overwhelming firepower?
ABSOLUTELY - it is a (genuine) superpower by all accounts. But how a US government would politically justify such a move against an external threat unless it presents a significant threat to survival of US and cannot be discouraged through diplomacy? North Korea might be perceived as a threat of such caliber [at some level within US] but it have not targeted US [assets] in the surrounding areas in spite of its hostile political posture.
Furthermore,
social media have emerged as a powerful element in current times that can influence political decisions to a certain extent. This element was missing during WW2 and military forces [of any nation] could go to any length to achieve its goals back then. However, in current times, indiscriminate bombings in urban areas and genocide cannot be politically justified and concealed unless something happens that is really bad and creates an opportunity for political will to carry out such actions.
On top of everything, US is a
democratic nation and it is unlikely to be as brutal (and/or worse) in its actions as a
hardened dictatorship [is normally expected to be] without sufficient external pressures that may fuel such sentiments within American public. Remember global (and domestic) reactions to
Abu Ghraib torture scandal in Iraq?
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On a separate note, some people mistakenly assume that US military forces are not efficient in combat operations, they are normally brilliant in execution of (any) military operation. Performance of US military forces in major battles of
Operation Iraqi Freedom and
Operation Enduring Freedom are actually studied by military forces of many nations in the world (often quietly) to learn lessons from them and emulate the professionalism of US military forces in similar situations [should they ever arise]. For example, major US military operations in urban environments of Iraq serve as a benchmark of brilliance and excellence in tactics and techniques for military forces in such environments [worldwide]. Examples include
Thunder Run in Baghdad and
Operation Phantom Fury in Fallujah.
US military forces are actually trend-settlers in warfare techniques. I can cite numerous examples.
