Going back into the Clinton years and before it was assumed in the US intelligence community that Saddam Hussein's Iraq had some continuing chemical weapons program. Post invasion we learned that was wrong. But chemical weapons, as hideous as they are, were never enough to spur the fears the Bush administration used to justify the invasion of Iraq. They needed much more. They needed ties to terrorism and an Iraqi nuclear weapons program or biological weapons program.
The US intelligence agencies saw little evidence for either - especially an active nuclear weapons program or ties to international terrorism, particularly ties to 9/11 which Bush administration officials were eager to find.
So there was a year long process in which the Bush administration pressured the CIA and other intelligence agencies to come up with evidence. To some degree, the CIA did do that, but never as much as the White House and key appointees at the Pentagon wanted. But through the roughly 18 months between the 9/11 attacks and the invasion of Iraq there was a running war within the US government. Indeed, CIA and other intelligence agency officials ran an on-going effort to prevent the President and other high level administration officials from publicly disclosing misleading or erroneous claims about US intelligence. One of the biggest fights was over statements about intelligence about nuclear weapons - uranium, certain high tolerance parts which could be used for nuclear weapons.
This is at best a qualified defense of the CIA. To a real extent, they gave in to the Bush administration. What we are seeing now is people being upset with President Obama (with some reason) for not sufficiently pushing or sounding the alarm about what the intelligence agencies were finding. In other words, perhaps erring on the other side of the equation. Even more importantly, once President Trump is in office and his extremely partisan CIA nominee, Mike Pompeo, you can guess how aggressively they will look to get to the bottom of Russian state subversion of the 2016 election.
Trump's claims about the US intelligence community and weapons of mass destruction is about as solid as his oft-repeated claim that he opposed the US invasion of Iraq.