Brazilian troops have started putting out large swathes of fires in the Amazon rainforest.
It comes as the BBC found the record number of fires coincided with a sharp drop in fines for environmental violations.
It is not known how many of the fires, which have increased by 84 per cent compared with the same period last year, were set deliberately but critics have accused president Jair Bolsonaro's administration of "green lighting" the destruction of the rainforest through a culture of impunity.
Around 44,000 troops will be available for "unprecedented" operations to put out the fires, and forces are heading to six Brazilian states that asked for federal help.
The military's first mission will be carried out by 700 troops around Porto Velho, capital of Rondonia.
It will use two C-130 Hercules aircraft capable of dumping up to 12,000 litres of water on fires.
At the G7 summit he is hosting in Biarritz, French president Emmanuel Macron indicated he could block an EU trade deal with Brazil and its neighbours over Mr Bolsonaro's country's handling of fires in the rainforest.
UK Prime Minister Boris Johnson has said he is "horrified" by the devastation in the Amazon, but warned against using the destruction of the rainforest to block trade deals.
Irish premier Leo Varadkar has also said he would not back the EU-Mercosur agreement if Brazil did not honour its environmental commitments.
But Mr Johnson cautioned against the approach at a "very difficult time for global free trade".
Mr Johnson said he is "horrified by what is happening in the Amazon basin" and the UK "stands ready to help in any way that we can to extinguish the fires and to protect habitat and biodiversity".
But he added: "I think I'd be reluctant to do anything, at this very difficult time for global free trade, to cancel another trade deal."
