The word 'invasion' depends on whom you ask. According to most Dominicans, the US 'invaded' the DR twice, once in 1916 to 1924 and once in 1965 to 1966.
Most history books -- especially American ones -- refer to these events as an occupation or intervention. It depends a lot on who's side you're on and how you saw the reasons for the events. In both cases the US sent Marines to help stabilize the country. In both cases Dominican rebel groups unsuccessfully fought a guerilla war against them, with lives being lost on both sides.
the US flag over the Ozama fortress in 1916
In both cases the effect of the incursion was positive, helping the Dominican government establish itself after a period of revolts and bringing changes to the DR (and Haiti) economy.
However, the point of contend it the reason. In both cases the reason for the invasions was not necessarily to 'help the DR' but rather to protect American investment interests. DR issues came secondary and so the support the guerrilla groups had was overwhelming. One other negative aspect of the occupation was the result: upon departure after the 1916 occupation, the American forces left behind in charge Dominican officers/politicians they thought fit. One such high-ranking officer was Leonidas Trujillo, who later became a Hitler-like dictator thanks to US support.
The 1965 intervention once again was over US interests. In this case political as well as financial, to avoid the DR to become a second Cuba after the fall of Trujillo. The irony is clear.
Honduran Inter-American Peace Force soldiers arrive in the Dominican Republic, 1965
To this day opinions are split regarding the nature of the interventions, but for the most part they are seen in the DR as invasions.