One can often hear politicians, diplomats, and their ilk using the word "illegal", like the recent references in an EU report to "illegal Israeli settlements in the West Bank".
And expulsion of some Palestinians from the Coastal Palestine to make room for the State of Israel in 1948 was also "illegal", and so is the continued occupation of the West Bank after the 1967 war.
It is also a generally known fact that the war against Iraq by the USA had no valid reasons, but was justified by false arguments and political demagogy. All of which is "illegal". And the same is true of the USA/NATO invasion of Afghanistan.
Imprisonment and torture without valid reasons, as it was done by the USA in Guantanamo, is also "illegal", and so are the "drone" attacks by the USA in Pakistan, Afghanistan, and other countries.
But what does "illegal" mean?
"Illegal" means "against the Law".
But, if something is against the Law, then the Law should stop such illegal activity, punish its perpetrators, and rectify any damage done by the illegal acts.
So, why are "illegal" acts not stopped by the Law?
Why the criminals are allowed to continue with their illegal acts with impunity?
Because whatever Law there existed has fallen into disuse, or was corrupted and perverted by politicians to suit their political objectives, and people, instead of behaving lawfully, commit crimes, and, instead of resolving their disputes by applying Law, call each other names, fight each other, or play on each other nasty tricks.
Such human behaviour is not new, but from earliest times there appeared people who understood that humans can rise above such behaviour by establishing Law based on Justice.
To make people accept Law based on Justice was not an easy task, but whenever such Law was established civilizations prospered, but when it was abandoned barbarity reined, and civilizations collapsed. Thus, the ancient Laws of Moses lead to a group of small tribes establishing a civilization, which decayed when the people abandoned the Law. At the height of the Roman Empire the Roman Law reached the height of its development, but as corruption spread, the Roman Empire collapsed. The English Law attained the height of its development when the British Empire reached the peak of its growth, but then decay set in and corruption followed.
The reason for the decay of the civilizations of the past is that having achieved the height of development they never succeeded in applying the Law based on Justice to all Mankind, but restricted it to their own groups, while treating other groups as "inferior" and oppressing those "inferior" groups. This lead both to resistance of the oppressed on the one hand, and to loss of moral integrity by the ruling groups themselves, corruption of their institutions, and of Justice based Law giving way to lawlessness and barbarity, similar to what can be observed today.
But when human conditions deteriorate, people begin to look for ways to improve their conditions and they re-discover the need for Rule of Law. The latest attempt to establish Laws for resolution of disputes between nations was made after the Second World War, and the Organization of the United Nations was established for that purpose.
But the UN was established by the "Great Powers" who won the Second World War, and it is not effective when International Law is violated by the "Great Powers" themselves.
On the other hand since the end of the Second World War many new nations have emerged and joined the UN organization. So it is now for these other nations to reform the UN so it can deal with the criminal behaviour of the lawless "Great Powers" which has plunged the world into the present lawlessness and barbarity.