Trump was right about Bush’s invasion of Iraq. It was criminally stupid and empowered Iran. It also led to the destruction of the Christian community in Iraq. And all this started years before Obama. In fact, his election was an unfortunate result and backlash to Bush’s Iraq disaster.
I see your point that Iran benefited from Bush taking out Saddam - there are bound to be many effects from such a big move. The enemy gets a vote in how things fall out, and we have many enemies in the Middle East.
I still think that Saddam needed to go - he was bent on revenge against America, and nurturing the capability to do it. The big failure in my view, was not installing a pro-American regime in his place. That would have protected the Christians and been a blow to Iran (free Shi’tes, right on their border, mixing with their oppressed population in the Shi’ite holy sites in Karbala and Najaf). Long term US bases in Iraq would have extended US power and constrained Iran. If we had Iyad Allawi as Prime Minister, and protected them from Iranian subversion, Iraq would be an important US asset today, and ISIS would never have had a chance.
Bush mismanaged that critical transition, but Obama massively scuttled it. He released all the worst actors from the Camp Bucca Confinement Facility near Basra in a flood, and publicly announced that he would withdraw all American forces in one year. That was deliberately throwing Iraq to the dogs. It was deliberate sabotage of US interests. It was malicious. Iraq’s cities lie in ruins and mass graves are stuffed because of the chaos he unleashed.
The day that the last US unit crossed the border, PM Maliki charged the Sunni Vice President with treason (punishable by death), causing him to flee the country, excluding Sunnis from power, and fundamentally institutionalized sectarian conflict.
The mass destruction of the Christian (and Yezidi) communities was predominantly at the hands of ISIS, which only came to power as a champion of estranged Sunnis. All communities experienced losses during the Bush years, but there is no real comparison with the genocide and ethnic cleansing that Christians experienced at the hands of ISIS during the Obama Administration.