due to = because of. against = towards, for, to.

"He is DUE to stress the importance of security." In that sentence, "due" is an adjective that means "expected." So: "He is EXPECTED to stress the importance of security."

In this situation it means Obama is UNDER AN OBLIGATION OR AGREEMENT (his speech is already prepared based on earlier meetings with his advisors) AND READY TO SPEAK (at the appointed time) about Russia. It has both the meaning of some sort of pre-arranged obligation and a time factor (soon). Other examples: The chairman is due to speak at the end of the meeting; The police are due to arrest the criminal as soon as the Crown Prosecution Service agrees. I'm due to pay the gas bill (I should pay it, it's ready to pay). Compare that last one with "The gas bill is due." (it will arrive soon). "The gas bill is due to be paid (the bill may or may not have arrived, but I know I have to pay it soon). Actually, all these are very close in meaning. As to Obama, he is due to make a speech (at a particular time), in it he is due (expected/ready/obligated) to announce his disapproval of Russia. Against, in this case, simply means he will warn Russia NOT to cause problems (in terms of stability). Examples: Sally warned Peter against swimming in the polluted river; The police warned the public against taking the law into their own hands. I warned him against doing anything stupid (I warned him NOT TO DO anything stupid)..

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