What do Reformed Christians understand by God's foreordination?

The word ‘foreordained’ is usually a translation of a Greek word meaning “to know beforehand.” Speaking about the birth of Jesus, 'foreordained' is used in that sense in 1 Peter 1:20 (KJV):

Who verily was foreordained before the foundation of the world, but was manifest in these last times for you.

The meaning of ‘foreordained’ becomes clearer in Ephesians 1:5 (KJV):

Having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will.

In other words, God has “predetermined” or “marked out beforehand” those who would be adopted into His family.

You ask if this means that God wanted and willed evil events (like wars) to happen and that men were powerless to stop evil events (like wars) to happen. The answer to that question is NO and the explanation is clear when all eight points in Chapter 3 – Of God’s Eternal Decree – are taken into account. Section 1 makes it clear that God is not the author of sin, neither is the will of men constrained. The liberty or contingency of second causes is not taken away, but is established.

Here is a Reformed Protestant understanding of the Westminster Confession, Chapter 3, Of God's Eternal Decree:

"Although God knows whatsoever may or can come to pass upon all supposed conditions; yet hath he not decreed any thing because he foresaw it as future, or as that which would come to pass upon such conditions."

It adds "(4) that this does not make God the author of sin, (5) that God does not force men to do what they do not want to do (in the way of sin), (6) that this does not destroy 'freedom' or cause and effect relationships (rather, it is the very basis upon which these exist), and (7) finally, that this sovereign plan of God is not 'conditioned upon' anything foreseen by him (which would make God dependent upon something outside himself)."

Page 31 makes this point, which can be applied to men starting wars which are evil (i.e. not out of need to defend from an aggressor). This shows that God does not cause people to start evil wars:

"Freedom may be defined as 'the absence of external coercion." If a man is not forced by any power outside himself to do that which is contrary to 'what he wants to do,' then we may properly say that he is 'free'. The wonder of God's predestination is that God does leave men free in this sense, even though he predestines everything that every man will ever do. Some people see the word 'freedom' in another sense, however, which is false in the extreme. They mean, by the 'freedom' of man, that man has the power to do good or evil at any moment of time. To say that a man is able to do good or evil, is very different from saying that a man is at liberty to do what he desires. We believe that man is at liberty but not ability to do what is right. For the truth is that man, while free from coercion from the 'outside' is not free from the control of his own nature. He who is evil by nature must of necessity produce corrupt fruit, Mt. 7:17-19). Just as we may say that God is good and therefore cannot do evil, so we may say that man (by nature) is evil and cannot (of himself) do good."

Source: "The Westminster Confession of Faith for Study Classes" by G.I. Williamson, (Presbyterian & Reformed Publishing, 1964)

Isaiah 46:9-11(KJV) declares the sovereignty of God:

Remember the former things of old: for I am God, and there is none else; I am God, and there is none like me, declaring the end from the beginning, and from ancient times the things that are not yet done, saying, My counsel shall stand, and I will do all my pleasure... I have spoken it, I will also bring it to pass; I have purposed it, I will also do it.

We might not like it, and we might not fully understand it, but we know that God has foreordained the way of salvation for “whosoever believes” in His Son, Jesus Christ (John 3:16–18, 36; Acts 10:43). God has foreordained that believers will be adopted into His family as children of God (John 1:12) and that He will take the responsibility of transforming us into the likeness of Jesus (Romans 8:29–30). God has foreordained that all those born again by grace through faith will spend eternity in heaven with Him (John 3:3; 17:3; Ephesians 2:8–9). We can rest in the knowledge that we have a Father who is in charge of His universe and that His plan is perfect.

What shall we say then? Is there unrighteousness with God? God forbid! (Romans 9:14 KJV)