In the past 10 years, analysts say the United States has provided Pakistan with about $20 billion in military and civilian aid, making it the third-largest recipient of U.S. security aid and reimbursements after Afghanistan and Israel. Both sides also have cooperated in targeting Taliban and al-Qaida-linked militants sheltered in Pakistan's semi-autonomous and remote northwest.
However, anti-Americanism remains high in Pakistan, due to the unpopularity of U.S. drone strikes against militants on Pakistani soil and Washington's insistence that Islamabad do more in the fight. Meanwhile, reports of suspected Pakistani assistance for militants have caused some U.S. lawmakers to insist on suspending financial aid to Pakistan.
U.S. and Pakistani officials describe their relationship as a "bad marriage, where divorce is not an option" due to their shared threat of extremism.
But the secret U.S. commando raid deep into Pakistani territory earlier this year to kill al-Qaida leader Osama bin Laden took this distrust to a new level.
Here is a look at some of the key events in 2011:
May 2: A covert U.S. commando raid deep into Pakistani territory kills Osama bin Laden. Pakistan responds with outrage that it was not informed beforehand and anger because of what it saw as a U.S. violation of its sovereignty. Some U.S. lawmakers call into question Pakistan's relationship in the war on terrorism after learning the al-Qaida leader had been hiding in a Pakistani military garrison town near the capital, Islamabad, apparently for years.
Pakistan's military later warns that any future U.S. raids on Pakistani territory will result in a review of military and intelligence cooperation with the United States. U.S. officials say they have not seen any evidence that top Pakistani officials knew bin Laden was hiding in their country.
May 16: U.S. and Pakistani officials start a series of meetings in Islamabad to try to repair strained bi-lateral relations.
June: Pakistani security officials say that about 90 of the approximately 130 U.S. trainers in their country had been sent home. Pakistan authorities also reportedly arrest four informants who helped U.S. intelligence agents find bin Laden. Pakistan's military later rejects a U.S. media report that says a cell phone found during the bin Laden raid contained contacts to a militant group that has "strong ties" to Pakistan's military spy agency the ISI.
July 10: U.S. officials announce their decision to withhold $800 million - a third of U.S. military aid to Pakistan - in response to Islamabad's decision to expel American military trainers and put limits on visas for U.S. personnel.
Sept 5: Pakistan's military says it arrested a senior al-Qaida leader and two colleagues with help from the U.S. Central Intelligence Agency.
Sept 14: U.S. Defense Secretary Leon Panetta warns Pakistan that the United States would "do everything we can" to defend American forces in Afghanistan from Pakistan-based militants.
Sept 21: The United States agrees to limit the number of military personnel stationed in Pakistan. The new agreement between the two countries cuts the number of U.S. troops allowed in Pakistan by half, to between 100 and 150, and the number of elite special operations trainers from around 140 to as few as a dozen.
Sept 22: Outgoing U.S. military chief Admiral Mike Mullen calls the al-Qaida-linked Haqqani network a "veritable arm" of Pakistan's ISI. He accuses the military spy agency of helping the Haqqani network launch attacks in Afghanistan, including an assault on the U.S. Embassy in Kabul.
The White House then calls on Pakistan to break any links it has with the Haqqani network and accuses Islamabad of not taking action against the group's safe havens believed to be located in northwestern Pakistan.
Sept 29: Pakistan's spy chief admits that his agency maintains contacts with the Haqqani network, but does so not in order to wage a "proxy war" against the U.S. interests in neighboring Afghanistan, but for the purpose of seeking peace.
Oct 6: Pakistan charges a local doctor with treason after he allegedly helped the United States with its bin Laden raid.
Oct 18: Pakistan's army chief warns the United States against taking any unilateral military action in the North Waziristan tribal region, believed to be a hub of the Haqqani network.
Oct 20: U.S. Secretary of State Hillary Clinton tells Pakistan it "can either be helping or hindering" efforts to "create a strong foundation for an Afghanistan free from interference, violent conflict and one that has a chance to chart its own future."
Nov 23: Pakistan names a new ambassador to the United States after the previous one stepped down following claims that he appealed for Washington's help in reining in Pakistan's powerful military following the bin Laden raid.
Nov 26: Pakistan says it plans to review its complete relationship with the United States and NATO in response to a deadly cross-border NATO airstrike.
