The Bush administration advertised Iraq as an intervention to stop "weapons of mass destruction," which the Bushies let the public think that meant Saddam Hussein with nukes, while Bush and his people were confident they'd at least find some old canisters of mustard gas. It turns out Saddam Hussein really had destroyed his chemical stockpiles; he never was very far advanced on nuclear enrichment. So the rationale of the war fell apart. Then the Bushies said it was fought for democracy, but you couldn't call what is happening in Iraq politically exactly democracy. Then ISIL arose and took 40% of Iraq away from the Shiite government in Baghdad, and the US had to go back in to help destroy the terrorist organization.
Bush borrowed the money to pay for his wars, putting up the deficit and contributing to the Bush Depression.
So that's a pretty clear failure, and it is difficult to think of even one benefit to the US of having gone over there. The third of vets who still think it was a good idea don't, I think, want to admit that Bush and the GOP were on a wild goose hunt.
Afghanistan has gone even worse, with the Taliban having taken back 50% of the country and the US seeking talks with them.
In all these conflicts, there was no clear grand strategy, no clear, achievable goals, and nothing that looked like victory.
The old saying is that victory has a thousand fathers but defeat is an orphan. With the exception of the fight against ISIL in Syria, these were defeats. Even in the case of Syria, when it is all over the US will likely have defeated ISIL so that the Baath government of dictator Bashar al-Assad, and his backer Vladimir Putin, can pick up the pieces. Most vets wouldn't view that outcome as having been a particularly worthwhile sacrifice.
One thing seems clear. If a majority of Republican veterans can't be found to uphold the value of the Iraq, Afghanistan and Syria Wars, we damn sure ought not to be starting any new conflicts in the Middle East that are doomed to run up our national debt, bankrupt us, devastate our economy, and produce more disillusionment -- even among the most gung-ho section of the population.
Yes, I'm saying we should just skip the proposed Iran War and avoid having to read 20 years from now about how few who fought in it thought it was a good idea.