Since 2005, Pakistan has ordered $4.5 billion worth of American weapons and military equipment. This includes a wide variety of gear. The big ticket items included 36 F-16 fighters, and upgrades to existing Pakistani F-16s. Another big item is M109A5 155mm self-propelled howitzers. There are also a lot of spare parts, military supplies and technical services. Pakistan is supposed to only use this stuff for counter-terror operations or national defense. Thus if Pakistan gets into another war with India, and uses some of the American gear for offensive operations, they could be declared in violate of their agreement with the U.S., and be cut off from spare parts and other support needed to keep the American weapons working.
The biggest problem, however, has been the endemic corruption in Pakistan. Much of the aid was not only diverted from counter-terrorism, to other functions (like war with India), but was simply stolen. Continued aid is contingent on Pakistan accepting audits and controls on these funds, and the Pakistanis do not like this one bit. It is feared that, as happened in Iraq and Afghanistan, that the auditors, working for the Americans, might be threatened or attacked.