The plunge around 2001 wasn't caused by 9/11, and was orchestrated a bit before. Since Clinton managed to cut a lot of spending (both due to his governance and global economics) and reduce some tax avoidance schemes, the US budget had a surplus, so the Bush admin implemented a lot of tax cuts. 9/11 and the following wars didn't had any positive impact, but without the Bush tax cuts, the US budget would still have a surplus in 2003 before the Iraq war