He already invaded Ukraine. In 2014.
In 2016, Trump promised the house Russia from Ukraine once he's elected.
Instead, in 2017, first week in office, Trump attempted to recind all sanctions placed against Russia and return properties a Russian agent in the US confiscated by Obama.
Putin was waiting till Trump's second term to invade the rest of Ukraine because Trump was supposed to pull us out of NATO, and would have done nothing in response.