Why did fighting continue in iraq after president bush declared the major combat was over

President Bush announced the end of major combat operations, not that the war was over.

Explanation:

Neither President officially said the war was over. And the administration in a bit of cockiness did make a variety of statements about how the war was over (the "mission accomplished" banner, a speech by Rumsfeld, etc.).  The Bush administration did not anticipate or plan for any kind of real insurgency and at the time of the invasion, there was no al-Qaeda presence in Iraq so the continued fighting came as a major surprise.

President Obama announced the end of US combat involvement in Iraq.  Technically that was true--the SOFA (US status of forces agreement) had expired and the only elements we were allowed to have in Iraq at that point were trainers, security (for US personnel and our embassy) and support (for instance, the Iraqi air force was almost non-existent).  But no authorized combat units.

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