If you are thinking ground troops as in expeditionary forces - think the International Brigades or the Condor Division - there weren't.

What there was, was a military advisory mission, composed of five Czech officers, sent to provide training and assistance to the Bolivian Army. So, how did they get there?

During the conflict, the Peruvian and Bolivian high command had several foreign advisers, that at times took actual command of the forces. This was standard practice in Latin America.

In Bolivia's case, the most (in)famous was General Hans Kundt, a German WWI veteran, that had trained the Bolivian Army in the late 1900, and who returned after the War. By all accounts a competent administrator and trainer, and popular with the troops, he took command of the Bolivian Army during the Chaco War.

He... wasn't too successful. The Gran Chaco is a heavily wooded area, but very dry. Bolivian armor - especially purchased - could hardly operate in the thick scrub, and required water - already scarce - for cooling. Poor logistics made desertions a massive issue, as soldiers without rations or water desperately tried to flee. Air recon - Bolivia had one of the best air forces in Latin America - was regularly dismissed by him.

By December 1933, and after the second failed offensive at Nanawa, the encirclement at Campo Grande and posterior Paraguayan counter attack and especially the disaster at Campo Vía - and facing widespread distrust from Bolivian officers and politicians - he was relieved of command.

After that, Bolivian president Salamanca promoted General Peñaranda to the post previously held by Kundt. To advise and assist him, he hired a Czechoslovak military mission. Czechoslovakia - which had a huge arms industry - saw Bolivia as a potential customer, as well as an opportunity to try their products in extreme conditions, and sent the mission.

As the Czechoslovak foreign affair minister at the time was the Society of Nations' arbiter in the conflict, the mission had to be camouflaged. The advisers were sent to reserve corps, and traveled to Bolivia as civilians, and they were initially quite distrusted by the Bolivian officers, who didn't have much sympathies for Salamanca either.

Despite opposition by the military, the advisers toured the fronts, and provided their reports to the Bolivian high command. Eventually, one of the advisers was assign to train infantry units, another artillery forces, one was assigned to the Chief of Staff, and one sent to train and advise the Engineer corps.

The situation for Bolivia would only worsen. In the middle of a political crisis - President Salamanca was removed by the military - a War Council was created, where Pacek was given a place. He was instrumental in creating the last Bolivian army, which managed to score some of the few successes the war had for Bolivia.

Hopefully this will sate some of your curiosity! There's a lot of literature in Spanish on the war, not so much in English, but I can try to find some if you want to read up on it.

Could you expand on your question? Czechoslovakia did not have ground troops in the Chaco War (1932-1935). They did not participate in direct actions in the war as combatants. If you could give me some more information about where you've heard about this, I could look into it.

A contracted Czechoslovakian military mission (i.e. military advisers) was present to help the Bolivian army to reorganize and to advise Bolivian generals as to the best conduct of the final stages of the war. This had practically no impact on the end of the war and as Bolivian general José Félix Estigarribia writes in his memoirs of the war, most Boivian generals looked upon this foreign 'intervention' with suspicion seeing as past foreign military missions had not helped them turn the tide of the war.