I think ‘ensure’ might be one of the most misused words in the English language. At least 90% of the time I see it used, it is being seriously misapplied. Cariboo altering their migration patterns in response to climate change are not ‘ensuring’ their survival. At best, the behaviour makes their survival more likely. Similarly, new laws cannot ensure that children will be protected from sexual predators. At best, they will make such occurrences less frequent. Military and police power can never ensure an end to terrorism.
In short, the verb ‘to ensure’ should only be used in situations where genuine certainty is being produced. Generally, this is only the case in matters that are strictly logical or those that are strictly empirical. For instance, the fact that all bats are mammals ensures that there are no non-mammal bats. Similarly, the fact that matter attracts other matter ensures that very massive bodies will be roughly spherical. In terms of actions, those where something is ‘ensured’ are those where an inescapable cause-and-effect relationship exists: the failure of the O-ring on the Space Shuttle Challenger ensured that the vehicle would fail catastrophically. Virtually nothing in politics is certain and, as such, political pledges to ‘ensure’ things are usually misleading or empty.