Yes, you are to be blamed if it happens again. But not blamed for keeping borders open (the rest of the world keeps its borders open yet terrorist acts happen only in a few select countries for very explainable reasons). But yes, you need to them blame yourself for bigotry and hatred towards the rest of the world. You can't continue to support Israel without reservation and completely ignore Palestine. You can't continue to give a shit about anyone in the Middle East and only act in order to maximize your oil relations, you can't interfere here, you can't interfere there and tell people you are moving in to promote "peace and democracy" while in reality you only extract goodies that benefit American businesses. You can't expect the rest of the world to be stupid and blind to the fact that you support a despot and dictator, eg Erdogan, but at the same time "promote" democracy. Its a contradiction and makes American policy untrustworthy.
I mean of course you can do all the above, but then you should not be surprised that you have suicide bombers that will make their ways into your country or that do not even have to enter your country but simply motivate American citizens to commit acts of violence. When a person or group or culture is cornered and left with no alternatives then you should not be surprised that there are individuals who are not afraid to die.
A certain group of people in the US to which you obviously belong believe all problems are solved via strong force, an aggressive stance, and show of military might. How much more wrong can you be? Has history not taught you that none of the past 30-40 years of military show of power by Americans have gotten them nowhere? If anything you have moved many parts of the world against you. America does not stand for freedom and democracy anymore, it now stands for a bunch of few selfish people on the top who bend over backwards to get a "deal" done that benefits themselves the expense of the majority of their own people and the people of the country that is targeted by American hegemony. Have you never wondered why the US makes no exceptions in its support for Israel. Whom do you think that benefits? You? A farmer? A non Jewish IT expert? Anyone who is not a Jew? (which make up more than 97% o the American population). No, it benefits the few Jews on the top in the US who hedge hog almost every position in media, news, movie production, corporate board rooms, financiers, ibankers, private equity heads,...I am by no mean anti-Semitic, I simply state a fact. Before you even attempt to stamp me as an anti-Semite I challenge you to provide facts that contradict my statements. Honestly, tell me who else benefits from the US-Israel relationship.
Why are you surprised the US and its allies, who joined in aggressive "democratization" in the Middle East, are under attack? Why are you surprised countries like Japan or Germany or Scandinavia, or Switzerland are not being targeted by radical islamists? Why are you surprised your great war on drugs got you nowhere? Even if you build a wall then the drugs will enter through tunnels and through submersible marine vehicles.
Maybe time to wake up and try to put yourself just for a minute into others' position? It's the demand side where things on the drug issue are solved; it's the attitude and working on deals that are beneficial to
both sides that make people outside the US take a friendly and supportive stance towards Americans. If I was American I would not dare to set foot into 1/3 of the world today. It is not gonna be pretty. What a shame for all the upright and honest Americans that work hard every day and do the right thing. What a shame they are often put into the same bucket as your kind because of the human tendency to generalize.
Back on topic.
If another 911 happens in America, WE are to blame for voting in losers who kept the borders open, refused background checks, and put their arm around every Jihadi.
We are to blame, folks.
You voted for 'em. Time to take responsibility. This isn't a fucking game.