There was an actual rather than "fake" war, so the implication of an actual army is a rational conclusion.
Winston could not definitely remember a time when his country had not
been at war, but it was evident that there had been a fairly long
interval of peace during his childhood, because one of his early
memories was of an air raid, which appeared to take everyone by
surprise. Perhaps it was the time when the atomic bomb had fallen on
Colchester. (1.3.12) (Book 1, ch 3 p 12)
Also fighting on land, though if this represents a civil war or not is open to question.
Since about that time, war had been literally continuous, though
strictly speaking it had not always been the same war. For several
months during his childhood there had been confused street fighting in
London itself, some of which he remembered vividly. But to trace out
the history of the whole period, to say who was fighting whom at any
given moment, would have been utterly impossible, since no written
record, and no spoken word, ever made mention of any other alignment
than the existing one. ((1.3.16)Book 1, ch 3 p 16)
Tanks and planes indicate an army ...
On the sixth day of Hate Week, after the processions, the speeches,
the shouting, the singing, the banners, the posters, the films, the
waxworks, the rolling of drums and squealing of trumpets, the tramp of
marching feet, the grinding of the caterpillars of tanks, the roar of
massed planes, the booming of guns – after six days of this, when the
great orgasm was quivering to its climax and the general hatred of
Eurasia had boiled up into such delirium that if the crowd could have
got their hands on the 2,000 Eurasian war-criminals who were to be
publicly hanged on the last day of the proceedings, they would
unquestionably have torn them to pieces – (2.9.3)
There was also a scene where Winston saw a number of Eurasian prisoners of war being unloaded from transport by soldiers. Can't find the page reference at the moment. There is another clue (thanks to @Thunderforge for this comment): Winston volunteers at a munitions factory.
Orwell had a purpose in not being explicit. 1984 is a master work, in part due to the writing style used and things left unsaid. It can also be a different read based on what you bring to the table when you read it.
A side note: authors can work long and hard to get the right "voice" for a work, and for sections of a work. It can be a difficult creative process.
I read it about 40 years ago for the first time.
I read it 20 years ago for the second time. (After more life experience and after some harsh training about what being a captive of a totalitarian state holds as a challenge). My life experience changed the reading experience.
The other point is context: put yourself in Post War (WW II, 1950ish timeframe) London and also the 1950ish "world" -- that's context. It is my read that he was in part commenting on the Cold War and on the 20th century's rise of "the warfare state." England had in Orwell's lifetime dipped into the warfare state twice for two world wars. So too had other modern nations. As a keen observer of both politics, propaganda, and political rhetoric (I've a collection of his essays that is excellent reading) his expertise informs the style of his book.