The infosec industry are extremely skeptical of attribution especially coming from government - it took a long time for a higher-certainty attribution to be established in the Sony and Bangladesh reserve bank cases, and there are still open cases of attribution in other incidents
Along with technical evidence, Wannacry also fits the North Korea m.o of financial incentive but also incentive to sow chaos[12], as well as how their attackers operate in Singapore and China and outsource some components.
On the contrary there is little to no evidence pointing anywhere else - meaning the second best probability of attribution for Wannacry is much further down the list.
The key difference there is that during the Iraq invasion there was zero corroborative evidence, the actual weapons inspectors said there was no evidence.
Further - the USA doesn't need a made-up pretext for war with North Korea. They're technically still at war and North Korea are in violation of UN Security Council resolutions and treaties (they routed around the UN for Iraq)
North Korea has sunk a ship[4], downed an airliner[5], attacked South Korean islands[6], tested nuclear weapons, developed a missile program, fired across the DMZ[7] (as recently again as today[8]), assassinated foreigners on foreign soil[9], been caught attempting to sell weapons of mass destruction[10], sold arms to Syria and others in violation of sanctions[11] and more. Either one of those is a stronger and not made-up pretext for an act of war - and one that would have much broader support than Iraq ever did.
[0] https://twitter.com/neelmehta/status/864164081116225536?lang...
[1] https://www.symantec.com/connect/blogs/wannacry-ransomware-a...
[2] https://blog.comae.io/wannacry-links-to-lazarus-group-dcea72...
[3] https://securelist.com/wannacry-and-lazarus-group-the-missin...
[4] https://en.wikipedia.org/wiki/ROKS_Cheonan_sinking
[5] https://en.wikipedia.org/wiki/Korean_Air_Flight_858
[6] https://en.wikipedia.org/wiki/Bombardment_of_Yeonpyeong
[7] http://edition.cnn.com/2017/11/21/asia/north-korea-defector/...
[8] http://www.bbc.com/news/world-asia-42435798
[9] https://en.wikipedia.org/wiki/Kim_Jong-nam
[10] http://www.smh.com.au/nsw/alleged-north-korean-agent-arreste...
[11] https://qz.com/962995/the-war-in-syria-has-been-great-for-no...