The cited context is inherently "flawed, awkward", since it's using the copula form [cannot] be to link singular our country and plural [the] victims. But we could resolve that with, say,...
We cannot be [the] victims of our circumstances.
...where grammatically / syntactically the article the is entirely optional. Statistically, it's not likely to be included, as can be seen by comparing the non-negated forms We are victims of circumstance (229 hits in Google Books) and We are the victims of circumstance (6 hits).
But semantically there's at least a potential difference in emphasis, in that including the article implies that the victims do actually exist (logically extrapolated from the principle that the definite article references something previously mentioned or contextually obvious).
A pedant might therefore suggest that Trump's inclusion of the implies that he accepts there are/will be victims of horrendous attacks (he just doesn't want Americans to be those1 victims).
A politically-motivated pedant might therefore go on to suggest Trump's choice of phrase here reflects an "isolationist" world-view (Trump wants to ensure America isn't a victim, rather than to prevent all such horrendous attacks worldwide, and thereby ensure no-one is a victim).
That's just clutching at straws, though. Both versions are fine, and to all intents and purposes they both mean the same thing.
1 Note that my those is a "determiner" with exactly the same implications as the in respect of the "referent does actually exist" argument here.