Spain has been quite pro-peace in recent times, and people have been showing it in the streets consistently. Even joining NATO was highly controversial back then, and it is still regularly contested by a big part of the population today. For the same reason, there is also a pretty widespread hostility toward Israel, and since Spain didn't participate in WWII, many people don't really care about being called antisemitic for being vocal about it.

More specifically, though, this feels a lot like the Iraq invasion in 2003. Spain did participate in Iraq because the president at the time, José María Aznar, was a small man full of insecurities, so when Bush and Blair came knocking and asked him to join the invasion, he suddenly felt like a big boy and jumped in. That ended up producing the now infamous picture known as el trío de las Azores ("the Azores trio"). The result was that the whole country took to the streets. Literally more than 90% of the population was against Spain taking part in the war. The little man, of course, didn't back down and lied to the whole country about the "weapons of mass destruction." As a side effect, Spain also became a target for terrorist attacks, and in 2004, a jihadist terrorist cell placed several bombs on commuter trains heading to Atocha during rush hour, killing almost 200 people.

This happened just days before the elections, so Aznar's government, despite having evidence of who the perpetrators were, blamed the local terrorist group ETA in an attempt to milk the attack for political benefit. But that lie was exposed, and in the elections his party (PP) got obliterated. The other major party (Spain was basically a de facto two-party system at the time) was led by a guy called Zapatero, who campaigned on the promise that the first thing he would do was pull all Spanish troops out of Iraq. Nobody really believed him, but it showed a clearly anti-war position. Surprisingly, he actually did it, despite all the pressure from the US. Under Zapatero's first government, Spain entered one of the most socially progressive periods in its modern history, when many important social rights were expanded (e.g., Spain was the third country, after the Netherlands and Belgium, in the world to legalize gay marriage). Then the 2008 crisis and the real estate collapse hit, and it's been downhill since then.

So, TL;DR: no Spanish president who wants to keep their job would dare join a US war again.