There have to be credible sources stating that there have been at least one of:

• battles between Russian military and defenders, in the target nation.

• Russian troops clearly are on the other side of the border. Even if the target country doesn't fight back, the presence of troops within their territory will treated as evidence for YES as an invasion

• Russian attributed bombs or missiles or drone strikes being treated as an open war w/Russia

• Other kinetic attacks.

Threats leading to concessions of territory or political union would not count because it's too hard to evaluate. If a union occurs without any troops across the border until the union is complete and it is no longer a separate nation, then that would not be YES. But if via betrayal or for other reasons, Russian troops are able to enter while the country is still considered independent, that would trigger the Invasion clause here.

Sporadic, accidental, non-continuing allegations aren't enough to trigger a YES. Issues like whether there really are troops, or where they really come from, will be resolved by waiting for better data and for things to settle down.

If the target country ceases to be via annexation, legal union, etc. it's very likely to be judged an invasion, unless this happening is overwhelmingly treated as appropriate, legitimate etc by major world powers (at least 4 of US, Japan, US, UK, France). This extended situation, if unclear, can delay judgement for up to 1 year past due date, however, the timespan we consider still never passes the due date; the waiting period is for time to discover information on the truthfulness of the claim at all times before it were due.